test : added explicit None input test for scanner normalize_severity

[tmdeveloper007]

Closes #1708.

Summary of What Has Been Done:
Added a single test test_none_input_returns_info to testing/backend/unit/test_scanners_base.py inside the TestNormalizeSeverity class.

The existing test_non_string_input test covers numeric input (123) returning "info", but None as an explicit input value was not tested. str(None) produces "None" which is not in the severity mapping, so the function returns "info". This behavior is now made explicit.

Changes Made:

def test_none_input_returns_info(self):
    """None is a common input in real pipelines — str(None)="None" is not in the mapping, so returns "info"."""
    scanner = _ConcreteScanner("task_none", None)
    assert scanner.normalize_severity(None) == "info"

Impact it Made:

• Documents the None input expectation explicitly
• Prevents accidental regression if the str() coercion logic changes

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.