Add test coverage for parse_json_fields edge cases

[tmdeveloper007]

Problem

parse_json_fields in backend/secuscan/routes_json_helpers.py has basic tests but lacks coverage for: non-existent fields, falsy non-string fields, and empty field lists.

Fix

1. Add 8 additional unit tests covering the missing edge cases.

Acceptance Criteria

• testing/backend/unit/test_routes_json_helpers.py extended with 8 new tests.
• All tests pass locally.