/detected_fields api returns different values based on frontend encoding

[trevorwhitney]

Describe the bug
When the frontend is configured to use protobuf encoding, the detected fields response return null if no parsers are found (ie. for structured metadata). When the frontend is configured to use json encoding, the detected fields response returns an empty array [] if no parsers are found.

To Reproduce
Steps to reproduce the behavior:

1. Started Loki (k218)
2. Ingest logs
3. Query detected fields for ingested logs (make sure the response will include some structured metadata fields)

Expected behavior
The value should be the same for either encoding type.