fix: Fix NPE in MetricTracer when MutateRowsRequest is invalid

[mutianf]

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Fixes #554 ☕️

[codecov]

Codecov Report

Merging #557 (6b35f8c) into master (2dea8ab) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #557      +/-   ##
============================================
+ Coverage     81.32%   81.39%   +0.07%     
- Complexity     1128     1131       +3     
============================================
  Files           106      106              
  Lines          7040     7040              
  Branches        368      368              
============================================
+ Hits           5725     5730       +5     
+ Misses         1117     1114       -3     
+ Partials        198      196       -2     

Impacted Files	Coverage Δ	Complexity Δ
.../v2/stub/mutaterows/MutateRowsAttemptCallable.java	91.08% <100.00%> (+2.97%)	19.00 <0.00> (+1.00)
...om/google/cloud/bigtable/emulator/v2/Emulator.java	59.83% <0.00%> (-0.82%)	14.00% <0.00%> (ø%)
...ud/bigtable/data/v2/stub/EnhancedBigtableStub.java	96.55% <0.00%> (+0.49%)	25.00% <0.00%> (+1.00%)
...d/bigtable/data/v2/stub/metrics/MetricsTracer.java	83.54% <0.00%> (+2.53%)	16.00% <0.00%> (+1.00%)

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[igorbernstein2]

lgtm!