Allow for empty Mahara submissions
This is necessary for cases where multiple plugins are enabled and the
student is has been given the option to submit some combination of e.g.
uploaded files and/or online text and/or Mahara views, but not necessarily
all of these things. The other plugins allow for this and the submission
is only rejected if all enabled plugins are empty.

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>