Consider Log lines

```
msg=hello subqueries=5
msg=hello subqueries=0
msg=hello
```

If you run following logql
```
{foo="bar"}|logfmt|subqueries!=0
```
it returns
```
msg=hello subqueries=5
```

But if you run following logql
```
{foo="bar"}|logfmt|subqueries!="0" # NOTE: "0" as string.
```
it returns
```
msg=hello subqueries=5
msg=hello
```

NOTE: it also returns log lines that doesn't contain `subqueries` label in the first
place.

This cause subtle error if used in metric queries using that `label` in unwrap.

e.g: following logql on above log lines returns
```
sum_over_time({foo="bar"}|logfmt|subqueries!="0"|unwrap subqueries[1m])
```

Returns `pipeline error: 'SampleExtractionErr'`. Because, the lines without `subqueries`
labels are not skipped during `ProcessLine` and during metric extraction, our extractor
sets this `SampleExtractionerror`.

Fixes: #6713

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>