utils: checkstyle.py: Initialise staged trailers

There are no possible Trailers for staged changes as the commit message
has not yet been written.

Initialise the empty trailers when the commit object is initialised.

Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

utils/checkstyle.py

@@ -253,6 +253,9 @@ class StagedChanges(Commit):
     def __init__(self):
         Commit.__init__(self, '')
 
+        # There are no trailers to parse on a Staged Change.
+        self._trailers = []
+
     def _parse(self):
         ret = subprocess.run(['git', 'diff', '--staged', '--name-status'],
                              stdout=subprocess.PIPE).stdout.decode('utf-8')