fix 'null' filters breaking logs page

svelte-ui/src/pages/LogsPage.svelte

@@ -25,9 +25,9 @@
   let min: Date;
   let max: Date;
 
-  let projectGroupId = localStorage.getItem("sl2.logspage.project_group_id");
-  let projectId = localStorage.getItem("sl2.logspage.project_id");
-  let itemId = localStorage.getItem("sl2.logspage.item_id");
+  let projectGroupId = localStorage.getItem("sl2.logspage.project_group_id") || "";
+  let projectId = localStorage.getItem("sl2.logspage.project_id") || "";
+  let itemId = localStorage.getItem("sl2.logspage.item_id") || "";
 
   $: [min, max] = range.calculate(now);