make branch parameter optional for /api/v1/repos/{owner}/{repo}/contents/{filepath} (#11067)

routers/api/v1/repo/file.go

 	//   "201":
 	//     "$ref": "#/responses/FileResponse"
 
+	if apiOpts.BranchName == "" {
+		apiOpts.BranchName = ctx.Repo.Repository.DefaultBranch
+	}
+
 	opts := &repofiles.UpdateRepoFileOptions{
 		Content:   apiOpts.Content,
 		IsNewFile: true,
 	//   "200":
 	//     "$ref": "#/responses/FileResponse"
 
+	if apiOpts.BranchName == "" {
+		apiOpts.BranchName = ctx.Repo.Repository.DefaultBranch
+	}
+
 	opts := &repofiles.UpdateRepoFileOptions{
 		Content:      apiOpts.Content,
 		SHA:          apiOpts.SHA,
 		return
 	}
 
+	if apiOpts.BranchName == "" {
+		apiOpts.BranchName = ctx.Repo.Repository.DefaultBranch
+	}
+
 	opts := &repofiles.DeleteRepoFileOptions{
 		Message:   apiOpts.Message,
 		OldBranch: apiOpts.BranchName,