Fix documented default of read_data in unittest.mock.mock_open

The documented signature showed read_data=None, but the actual default is the empty string -- Lib/unittest/mock.py, the function's docstring, and inspect.signature all agree. The code default changed from None to '' in 2012 (0dccf65); the docs were never updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: iamsharduld

Doc/library/unittest.mock.rst

@@ -2539,7 +2539,7 @@ Alternatively you can just use ``vars(my_mock)`` (instance members) and
 mock_open
 ~~~~~~~~~
 
-.. function:: mock_open(mock=None, read_data=None)
+.. function:: mock_open(mock=None, read_data='')
 
    A helper function to create a mock to replace the use of :func:`open`. It works
    for :func:`open` called directly or used as a context manager.