[3.13] gh-151443: Fix documented default of unittest.mock.mock_open's read_data parameter (GH-151444)

(cherry picked from commit 9ad6ba0)

Co-authored-by: Shardul Deshpande <iamsharduld@users.noreply.github.com>

Author: miss-islington

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.