fix: show "merge" form for empty PR (#39640)

fix #39637

routers/web/repo/pull_merge_form.go

@@ -112,7 +112,7 @@ func (prInfo *pullRequestViewInfo) prepareMergeBoxFormProps(ctx *context.Context
 	// if this pr can be merged now, then hide the auto merge
 	generalHideAutoMerge := prInfo.MergeBoxData.canMergeNow && allOverridableChecksOk
 	var mergeStyles []any
-	if pull.IsStatusMergeable() {
+	if pull.IsStatusMergeable() || pull.IsEmpty() {
 		mergeStyles = []any{
 			map[string]any{
 				"name":                  "merge",
@@ -176,7 +176,7 @@ func (prInfo *pullRequestViewInfo) prepareMergeBoxFormProps(ctx *context.Context
 	if len(mergeStyles) > 0 {
 		mergeFormProps["mergeStyles"] = mergeStyles
 		prInfo.MergeBoxData.MergeFormProps = mergeFormProps
-	} else if pull.IsStatusMergeable() {
+	} else if pull.IsStatusMergeable() || pull.IsEmpty() {
 		// no merge style was set in repo setting
 		prInfo.MergeBoxData.infoCommitBlockers.AddInfoItem(
 			svg.RenderHTML("octicon-x", 16, "tw-text-red"),