Prevent adding nil label to .AddedLabels or .RemovedLabels (#14623)

* Prevent adding nil label to .AddedLabels or .RemovedLabels

There are possibly a few old databases out there with malmigrated data that can
cause panics with empty labels being migrated.

This PR adds a few tests to prevent nil labels being added.

Fix #14466

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Add doctor command to remove the broken label comments

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>

models/consistency.go

@@ -305,3 +305,13 @@ func CountWrongUserType() (int64, error) {
 func FixWrongUserType() (int64, error) {
 	return x.Where(builder.Eq{"type": 0}.And(builder.Neq{"num_teams": 0})).Cols("type").NoAutoTime().Update(&User{Type: 1})
 }
+
+// CountCommentTypeLabelWithEmptyLabel count label comments with empty label
+func CountCommentTypeLabelWithEmptyLabel() (int64, error) {
+	return x.Where(builder.Eq{"type": CommentTypeLabel, "label_id": 0}).Count(new(Comment))
+}
+
+// FixCommentTypeLabelWithEmptyLabel count label comments with empty label
+func FixCommentTypeLabelWithEmptyLabel() (int64, error) {
+	return x.Where(builder.Eq{"type": CommentTypeLabel, "label_id": 0}).Delete(new(Comment))
+}