Assignment restore fails with deleted group

Steps to reproduce:

• Create a course
• Create a group
• Enrol 2 users in the course, 1 in the group, 1 not in the group
• Create an assignment, with group submission on, but do not require users to be a member of a group to submit
• Log in as each of the users and submit something for the assignment
• Log back in as admin and delete the group
• Backup + restore the course

Expected result:

• Backup restores successfully

Actual result:

• Error: "ERROR: duplicate key value violates unique constraint "mdl_assisubm_assusegroatt_uix"
  DETAIL: Key (assignment, userid, groupid, attemptnumber)=(2, 0, 0, 0) already exists.
  INSERT INTO mdl_assign_submission (userid,timecreated,timemodified,status,groupid,attemptnumber,latest,assignment) VALUES($1,$2,$3,$4,$5,$6,$7,$8) RETURNING id
  [array (
  'userid' => '0',
  'timecreated' => '1540804290',
  'timemodified' => '1540804297',
  'status' => 'submitted',
  'groupid' => 0,
  'attemptnumber' => '0',
  'latest' => 0,
  'assignment' => 2,
  )]
  Error code: dmlwriteexception"

Reason: during restore, mod_assign maps any groupids it can't find onto 0 (no group), but there is already a no group submission in the same backup, so this causes the unique key violation.

Suggested fix: skip restore of any assign submissions where the groupid is set but the group does not exist in the backup (as those submissions cannot be viewed anyway).