bpo-34886: Fix subprocess.run handling of exclusive arguments (GH-11727)

Fix an unintended ValueError from :func:`subprocess.run` when checking for
conflicting `input` and `stdin` or `capture_output` and `stdout` or `stderr` args
when they were explicitly provided but with `None` values within a passed in
`**kwargs` dict rather than as passed directly by name.

Lib/subprocess.py

@@ -459,12 +459,12 @@ def run(*popenargs,
     The other arguments are the same as for the Popen constructor.
     """
     if input is not None:
-        if 'stdin' in kwargs:
+        if kwargs.get('stdin') is not None:
             raise ValueError('stdin and input arguments may not both be used.')
         kwargs['stdin'] = PIPE
 
     if capture_output:
-        if ('stdout' in kwargs) or ('stderr' in kwargs):
+        if kwargs.get('stdout') is not None or kwargs.get('stderr') is not None:
             raise ValueError('stdout and stderr arguments may not be used '
                              'with capture_output.')
         kwargs['stdout'] = PIPE