Refactor AddParam to AddParamIfExist (#29834)

When read the code: `pager.AddParam(ctx, "search", "search")`, the
question always comes: What is it doing? Where is the value from? Why
"search" / "search" ?

Now it is clear: `pager.AddParamIfExist("search", ctx.Data["search"])`

routers/web/user/package.go

@@ -125,8 +125,8 @@ func ListPackages(ctx *context.Context) {
 	}
 
 	pager := context.NewPagination(int(total), setting.UI.PackagesPagingNum, page, 5)
-	pager.AddParam(ctx, "q", "Query")
-	pager.AddParam(ctx, "type", "PackageType")
+	pager.AddParamIfExist("q", ctx.Data["Query"])
+	pager.AddParamIfExist("type", ctx.Data["PackageType"])
 	ctx.Data["Page"] = pager
 
 	ctx.HTML(http.StatusOK, tplPackagesList)