Benchmark before:
```
goos: darwin
goarch: arm64
pkg: github.com/go-logr/logr/benchmark
BenchmarkDiscardLogInfoOneArg-10              	58466427	       21.36 ns/op	     32 B/op	      1 allocs/op
BenchmarkDiscardLogInfoSeveralArgs-10         	24470527	       49.62 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardLogInfoWithValues-10          	21938677	       47.06 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardLogV0Info-10                  	24796845	       47.45 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardLogV9Info-10                  	26069977	       49.02 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardLogError-10                   	23731986	       47.94 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardWithValues-10                 	50995291	       22.28 ns/op	     64 B/op	      1 allocs/op
BenchmarkDiscardWithName-10                   	1000000000	        0.4716 ns/op	      0 B/op	      0 allocs/op
BenchmarkFuncrLogInfoOneArg-10                	2684097	      433.9 ns/op	   1152 B/op	      7 allocs/op
BenchmarkFuncrJSONLogInfoOneArg-10            	2413437	      480.4 ns/op	   1168 B/op	      8 allocs/op
BenchmarkFuncrLogInfoSeveralArgs-10           	1261210	      953.2 ns/op	   1448 B/op	     17 allocs/op
BenchmarkFuncrJSONLogInfoSeveralArgs-10       	1000000	     1003 ns/op	   1448 B/op	     18 allocs/op
BenchmarkFuncrLogInfoWithValues-10            	1247379	     1027 ns/op	   1480 B/op	     17 allocs/op
BenchmarkFuncrJSONLogInfoWithValues-10        	1000000	     1109 ns/op	   1496 B/op	     18 allocs/op
BenchmarkFuncrLogV0Info-10                    	1000000	     1043 ns/op	   1448 B/op	     17 allocs/op
BenchmarkFuncrJSONLogV0Info-10                	1000000	     1100 ns/op	   1448 B/op	     18 allocs/op
BenchmarkFuncrLogV9Info-10                    	24429925	       48.76 ns/op	    176 B/op	      2 allocs/op
BenchmarkFuncrJSONLogV9Info-10                	24799878	       53.82 ns/op	    176 B/op	      2 allocs/op
BenchmarkFuncrLogError-10                     	1000000	     1010 ns/op	   1480 B/op	     19 allocs/op
BenchmarkFuncrJSONLogError-10                 	1000000	     1124 ns/op	   1496 B/op	     20 allocs/op
BenchmarkFuncrWithValues-10                   	4088592	      296.0 ns/op	    264 B/op	      8 allocs/op
BenchmarkFuncrWithName-10                     	34046577	       32.95 ns/op	     96 B/op	      1 allocs/op
BenchmarkFuncrWithCallDepth-10                	34601535	       32.77 ns/op	     96 B/op	      1 allocs/op
BenchmarkFuncrJSONLogInfoStringerValue-10     	2062101	      562.0 ns/op	   1200 B/op	     10 allocs/op
BenchmarkFuncrJSONLogInfoErrorValue-10        	2278339	      529.9 ns/op	   1200 B/op	     10 allocs/op
BenchmarkFuncrJSONLogInfoMarshalerValue-10    	2108040	      533.6 ns/op	   1200 B/op	     10 allocs/op
```

Benchmark after:
```
goos: darwin
goarch: arm64
pkg: github.com/go-logr/logr/benchmark
BenchmarkDiscardLogInfoOneArg-10              	59617632	       20.01 ns/op	     32 B/op	      1 allocs/op
BenchmarkDiscardLogInfoSeveralArgs-10         	26171325	       47.09 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardLogInfoWithValues-10          	25975759	       45.76 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardLogV0Info-10                  	25654935	       46.02 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardLogV9Info-10                  	25722668	       46.00 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardLogError-10                   	25181846	       45.97 ns/op	    176 B/op	      2 allocs/op
BenchmarkDiscardWithValues-10                 	53340940	       21.92 ns/op	     64 B/op	      1 allocs/op
BenchmarkDiscardWithName-10                   	1000000000	        0.4757 ns/op	      0 B/op	      0 allocs/op
BenchmarkFuncrLogInfoOneArg-10                	2887884	      416.5 ns/op	   1152 B/op	      7 allocs/op
BenchmarkFuncrJSONLogInfoOneArg-10            	2556847	      474.0 ns/op	   1168 B/op	      8 allocs/op
BenchmarkFuncrLogInfoSeveralArgs-10           	1280838	      957.4 ns/op	   1448 B/op	     17 allocs/op
BenchmarkFuncrJSONLogInfoSeveralArgs-10       	1000000	     1003 ns/op	   1448 B/op	     18 allocs/op
BenchmarkFuncrLogInfoWithValues-10            	1241484	      967.6 ns/op	   1480 B/op	     17 allocs/op
BenchmarkFuncrJSONLogInfoWithValues-10        	1000000	     1021 ns/op	   1496 B/op	     18 allocs/op
BenchmarkFuncrLogV0Info-10                    	1202900	      925.3 ns/op	   1448 B/op	     17 allocs/op
BenchmarkFuncrJSONLogV0Info-10                	1000000	     1021 ns/op	   1448 B/op	     18 allocs/op
BenchmarkFuncrLogV9Info-10                    	24424683	       48.45 ns/op	    176 B/op	      2 allocs/op
BenchmarkFuncrJSONLogV9Info-10                	24633237	       48.23 ns/op	    176 B/op	      2 allocs/op
BenchmarkFuncrLogError-10                     	1000000	     1066 ns/op	   1480 B/op	     19 allocs/op
BenchmarkFuncrJSONLogError-10                 	1072557	     1093 ns/op	   1496 B/op	     20 allocs/op
BenchmarkFuncrWithValues-10                   	4094355	      292.9 ns/op	    264 B/op	      8 allocs/op
BenchmarkFuncrWithName-10                     	36661557	       33.96 ns/op	     96 B/op	      1 allocs/op
BenchmarkFuncrWithCallDepth-10                	32008606	       32.65 ns/op	     96 B/op	      1 allocs/op
BenchmarkFuncrJSONLogInfoStringerValue-10     	2329975	      517.9 ns/op	   1200 B/op	     10 allocs/op
BenchmarkFuncrJSONLogInfoErrorValue-10        	2200308	      522.0 ns/op	   1200 B/op	     10 allocs/op
BenchmarkFuncrJSONLogInfoMarshalerValue-10    	2242267	      536.0 ns/op	   1200 B/op	     10 allocs/op
```

The performance is similar. Only a nil pointer check is added in the main path.

Signed-off-by: spacewander <spacewanderlzx@gmail.com>

Signed-off-by: spacewander <spacewanderlzx@gmail.com>