Make the setting of markdown systemtag a bit safer

Because previously, if for whatever reason the first
`note, status = self.simplenote.get_note(note_id)` failed then this call
would not work:
`note, status = self.simplenote.update_note({"content": content,
				    "key": note_id,
				    "systemtags": note["systemtags"]})`

because note["systemtags"] could be undefined