On tag/branch-exist check, dont panic if repo is nil (#21787)

fix a panic found in gitea logs

modules/git/repo_branch_nogogit.go

@@ -52,7 +52,7 @@ func (repo *Repository) IsReferenceExist(name string) bool {
 
 // IsBranchExist returns true if given branch exists in current repository.
 func (repo *Repository) IsBranchExist(name string) bool {
-	if name == "" {
+	if repo == nil || name == "" {
 		return false
 	}
 

modules/git/repo_tag_nogogit.go

@@ -15,7 +15,7 @@ import (
 
 // IsTagExist returns true if given tag exists in the repository.
 func (repo *Repository) IsTagExist(name string) bool {
-	if name == "" {
+	if repo == nil || name == "" {
 		return false
 	}