Fix error parsing a non-existant configuration file

If a file (e.g. ~/.gitconfig) does not exist, we get None
here rather than a string.  NoneType lacks rstrip() so we
cannot strip it.

Signed-off-by: Shawn O. Pearce <sop@google.com>

tests/test_git_config.py

@@ -39,5 +39,14 @@ class GitConfigUnitTest(unittest.TestCase):
         val = self.config.GetString('section.nonempty')
         self.assertEqual(val, 'true')
 
+    def test_GetString_from_missing_file(self):
+        """
+        Test missing config file
+        """
+        config_fixture = fixture('not.present.gitconfig')
+        config = git_config.GitConfig(config_fixture)
+        val = config.GetString('empty')
+        self.assertEqual(val, None)
+
 if __name__ == '__main__':
     unittest.main()