B107: Skip None values in hardcoded password detection - #1232

The B107 check was incorrectly flagging None default values as hardcoded
passwords in function definitions. This is a false positive since None is a
legitimate and commonly used within __init__ initialization

This change modifies the hardcoded_password_default function to:
- Skip None values in parameter defaults
- Handle both ast.Constant (Python 3.8+) and ast.NameConstant (Python 3.7-)
  representations of None
- Update documentation to clarify None handling behavior

Example of code that no longer triggers B107:
def __init__(self, auth_scheme, auth_password=None):
    pass

Fixes #PyCQA#1227

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