Fix label filter between all and no (#26557)

Regression of https://github.com/go-gitea/gitea/pull/25886.
# Description 
For `labelIDs`:

9afcb0e046/routers/web/repo/issue.go (L171-L174)
 - `nil` mean no filter
 - `[0]` mean `no label` filter

When `selectLabels == "0"`, labelIDs should be `[0]` rather than `nil`
 # Before:
 


https://github.com/go-gitea/gitea/assets/50507092/3dac5075-6da0-4769-ba20-48a56f4063c0


#  After:


https://github.com/go-gitea/gitea/assets/50507092/ff79fd4c-b02e-4dfb-9309-ae7851f4dcdb

Co-authored-by: Giteabot <teabot@gitea.io>

routers/web/repo/issue.go

@@ -177,7 +177,8 @@ func issues(ctx *context.Context, milestoneID, projectID int64, isPullOption uti
 		ctx.Data["AllLabels"] = true
 	} else if selectLabels == "0" {
 		ctx.Data["NoLabel"] = true
-	} else if len(selectLabels) > 0 {
+	}
+	if len(selectLabels) > 0 {
 		labelIDs, err = base.StringsToInt64s(strings.Split(selectLabels, ","))
 		if err != nil {
 			ctx.ServerError("StringsToInt64s", err)