MDL-41945 mod_assign: Properly check if submission is empty

Previous empty submission checks required the submission to
be saved to the database. This patch adds a new method to
submission plugins that lets them report whether the submission
is empty before it is saved.

mod/assign/upgrade.txt

@@ -3,6 +3,9 @@ This files describes API changes in the assign code.
 === 3.2 ===
 * External function mod_assign_external::get_assignments now returns additional optional fields:
    - preventsubmissionnotingroup: Prevent submission not in group.
+* Proper checking for empty submissions
+* Submission modification time checking - this will help students working in groups not clobber each others'
+  submissions
 
 === 3.1 ===
 * The feedback plugins now need to implement the is_feedback_modified() method. The default is to return true