fix(api): skip label lookup in ListIssues when no labels filter given (#39890)

strings.Split("", ",") returns []string{""} (length 1), not an empty
slice, so the previous len(splitted) > 0 guard was always true and never
skipped the label ID lookup, even when the labels query param was
absent. Check the raw trimmed param for emptiness before splitting,
mirroring the pattern already used elsewhere in this file.

Fixes: https://github.com/go-gitea/gitea/issues/39889

routers/api/v1/repo/issue.go

@@ -335,8 +335,8 @@ func ListIssues(ctx *context.APIContext) {
 	keyword := ctx.FormTrim("q")
 
 	var labelIDs []int64
-	if splitted := strings.Split(ctx.FormString("labels"), ","); len(splitted) > 0 {
-		labelIDs, err = issues_model.GetLabelIDsInRepoByNames(ctx, ctx.Repo.Repository.ID, splitted)
+	if labelsParam := ctx.FormTrim("labels"); labelsParam != "" {
+		labelIDs, err = issues_model.GetLabelIDsInRepoByNames(ctx, ctx.Repo.Repository.ID, strings.Split(labelsParam, ","))
 		if err != nil {
 			ctx.APIErrorInternal(err)
 			return