Fix ListBranches to handle empty case (#21921) (#22024)

Fix #21910
Backport #21921

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

routers/api/v1/repo/branch.go

@@ -252,21 +252,26 @@ func ListBranches(ctx *context.APIContext) {
 	//   "200":
 	//     "$ref": "#/responses/BranchList"
 
+	var totalNumOfBranches int
+	var apiBranches []*api.Branch
+
 	listOptions := utils.GetListOptions(ctx)
+
+	if !ctx.Repo.Repository.IsEmpty && ctx.Repo.GitRepo != nil {
 		skip, _ := listOptions.GetStartEnd()
-	branches, totalNumOfBranches, err := ctx.Repo.GitRepo.GetBranches(skip, listOptions.PageSize)
+		branches, total, err := ctx.Repo.GitRepo.GetBranches(skip, listOptions.PageSize)
 		if err != nil {
 			ctx.Error(http.StatusInternalServerError, "GetBranches", err)
 			return
 		}
 
-	apiBranches := make([]*api.Branch, 0, len(branches))
+		apiBranches = make([]*api.Branch, 0, len(branches))
 		for i := range branches {
 			c, err := branches[i].GetCommit()
 			if err != nil {
 				// Skip if this branch doesn't exist anymore.
 				if git.IsErrNotExist(err) {
-				totalNumOfBranches--
+					total--
 					continue
 				}
 				ctx.Error(http.StatusInternalServerError, "GetCommit", err)
@@ -274,7 +279,7 @@ func ListBranches(ctx *context.APIContext) {
 			}
 			branchProtection, err := git_model.GetProtectedBranchBy(ctx, ctx.Repo.Repository.ID, branches[i].Name)
 			if err != nil {
-			ctx.Error(http.StatusInternalServerError, "GetBranchProtection", err)
+				ctx.Error(http.StatusInternalServerError, "GetProtectedBranchBy", err)
 				return
 			}
 			apiBranch, err := convert.ToBranch(ctx.Repo.Repository, branches[i], c, branchProtection, ctx.Doer, ctx.Repo.IsAdmin())
@@ -285,9 +290,12 @@ func ListBranches(ctx *context.APIContext) {
 			apiBranches = append(apiBranches, apiBranch)
 		}
 
+		totalNumOfBranches = total
+	}
+
 	ctx.SetLinkHeader(totalNumOfBranches, listOptions.PageSize)
 	ctx.SetTotalCountHeader(int64(totalNumOfBranches))
-	ctx.JSON(http.StatusOK, &apiBranches)
+	ctx.JSON(http.StatusOK, apiBranches)
 }
 
 // GetBranchProtection gets a branch protection