Fix checks in PR for empty commits (#20290) (#20352)

Backport #20290

* Fix #19603
* fill HeadCommitID in PullRequest
* compare real commits ID as check for merging


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

services/pull/check.go

@@ -89,7 +89,7 @@ func CheckPullMergable(stdCtx context.Context, doer *user_model.User, perm *acce
 			return ErrIsWorkInProgress
 		}
 
-		if !pr.CanAutoMerge() {
+		if !pr.CanAutoMerge() && !pr.IsEmpty() {
 			return ErrNotMergableState
 		}