Delete by CQL rejects missing or empty CQL

Overview:

As a sysop or librarian using the DELETE by CQL API I might accidentally omit the query parameter or mistype it: quer=username==foo

How to reprocude:
Call PgUtil.delete by CQL with CQL query parameter that is null (= missing), empty or whitespace only.

Expected Results:
PgUtil.delete by CQL rejects the API call with a 400 HTTP status code and doesn't delete anything.

Actual Results:
PgUtil.delete deletes all records.

Additional Information:
This bug has already caused severe issues: MODINVSTOR-901