fix(api): skip label lookup in ListIssues when no labels filter given (#39890) (#39893)

Backport #39890 by @SHIVANSHGARG07

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

Co-authored-by: Shivansh Garg <shivanshgarg587@gmail.com>

routers/api/v1/repo/issue.go

@@ -337,8 +337,8 @@ 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