Fix make_pre_authed_request function to not fail when path arg is None.

The default value for the path arg to the function is None. However, if
the path arg is not set or is set to None an exception is generated when
the path value is passed to urllib.unquote(). The function doc states that
if path is not provided then the value in the env is used, so fix is to
implement this behaviour.

Change-Id: I085124acb6ef3cecb2375bb97d27996e0c6fd36e
Fixes: bug #1198149
Merge "Fix make_pre_authed_request function to not fail when path arg is None."