Fix 500 when pushing release to an empty repo (#29554)

As title. 
The former code directly used `ctx.Repo.GitRepo`, causing 500.

https://github.com/go-gitea/gitea/blob/22b4f0c09f1de5e581929bd10f39833d30d2c482/routers/api/v1/repo/release.go#L241

Author: lng2020

routers/api/v1/repo/release.go

@@ -4,6 +4,7 @@
 package repo
 
 import (
+	"fmt"
 	"net/http"
 
 	"code.gitea.io/gitea/models"
@@ -215,6 +216,10 @@ func CreateRelease(ctx *context.APIContext) {
 	//   "409":
 	//     "$ref": "#/responses/error"
 	form := web.GetForm(ctx).(*api.CreateReleaseOption)
+	if ctx.Repo.Repository.IsEmpty {
+		ctx.Error(http.StatusUnprocessableEntity, "RepoIsEmpty", fmt.Errorf("repo is empty"))
+		return
+	}
 	rel, err := repo_model.GetRelease(ctx, ctx.Repo.Repository.ID, form.TagName)
 	if err != nil {
 		if !repo_model.IsErrReleaseNotExist(err) {