Docs: behavior of ommiting value in promtail's json stage

[masa213f]

Describe the bug

In the json stage 's docs (/docs/sources/clients/promtail/stages/json.md),
the behavior of omitting expression's value is not specified.
Could you please clarify?

To Reproduce
Steps to reproduce the behavior:

1. Started Loki (SHA or version)
2. Started Promtail (SHA or version) to tail '...'
3. Query: {} term

Expected behavior
A clear and concise description of what you expected to happen.

Environment:

• Infrastructure: [e.g., Kubernetes, bare-metal, laptop]
• Deployment tool: [e.g., helm, jsonnet]

Screenshots, Promtail config, or terminal output
If applicable, add any output to help explain your problem.