promtail: fix handling of JMESPath expression returning nil while parsing JSON

[pracucci]

What this PR does / why we need it:

Currently, when a JMESPath expression - executed by the json stage - returns nil, the value stored in the extracted data map is JSON encoded, thus nil gets encoded into the string null.

Further stages working on the extracted data (ie. labels) will read that value as a string containing null, causing some weird side effects.

In this PR I propose to keep the nil value in the extracted map: the getString(nil) returns error, so subsequent stages working with string will skip it.

The reason why I didn't skip storing the nil in the extracted map (which is another option) is because then there would be any difference between a json stage expression returning nil and not having that expression at all in the config. Right now there would be no difference, but in the future may be handy to be able to differentiate them (ie. think about a stage re-encoding into JSON).

Example of the issue:

Currently, given this pipeline:

pipeline_stages:
- json:
    expressions:
      app:
      message:
- labels:
    app:

And this input log:
{"message":"hello"}

At the end of the pipeline there's a label app whose value is the string null.

Checklist

• Documentation added
• Tests updated

[cyriltovena]

LGTM