[Fix] Fix JSON emission for implicit arrays in libucl

[Fix] Fix JSON emission for implicit arrays in libucl

When emitting UCL objects with implicit arrays (multiple values for the
same key) as JSON, the emitter was producing invalid output by:
- Not printing the key before the array for non-first elements
- Missing comma separators between elements

This caused rspamadm configdump -j to output malformed JSON like:
  "actions": {...}[
instead of:
  "actions": {...},
  "group": [