Removing Labels via EditPullRequest API (#5348)

* added the ability to provide an empty array at the EditPullRequests API to remove all labels

Signed-off-by: Lucien Kerl <lucien.kerl@wuerth-it.com>

* Update pull.go

routers/api/v1/repo/pull.go

@@ -405,7 +405,7 @@ func EditPullRequest(ctx *context.APIContext, form api.EditPullRequestOption) {
 		}
 	}
 
-	if ctx.Repo.CanWrite(models.UnitTypePullRequests) && (form.Labels != nil && len(form.Labels) > 0) {
+	if ctx.Repo.CanWrite(models.UnitTypePullRequests) && form.Labels != nil {
 		labels, err := models.GetLabelsInRepoByIDs(ctx.Repo.Repository.ID, form.Labels)
 		if err != nil {
 			ctx.Error(500, "GetLabelsInRepoByIDsError", err)