chore: [k214] fix: panic when parsing and extracting JSON key values

[grafanabot]

Backport 5ef83a7 from #13790

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What this PR does / why we need it:

This fixes a panic when parsing end extracting values from JSON log lines with keys that consist entirely of whitespaces.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.