PATCH /entities/items/{item_id}/labels route correctly forbids to change a label to an empty string (which some Wikibase logic interprets as an instruction to remove the label) -- see T332739.

However, if provided an input that is just whitespace, the API ends up passing the empty label further down, resulting in removal.
This seems to be caused by the fact the input label "value" has leading and trailing spaces "trimmed". Checking for emptiness possibly happens prior to the "trimming" operation.

Example JSON Patch

{
  "op": "replace",
  "path": "/en",
  "value": " "
}

It seems the PUT /entities/items/{item_id}/labels/{lang_code} correctly rejects the "only whitespace" input