fix(ui): prevent infinite render loop in Tabs with routed tabs

When navigating to a URL that doesn't match any tab href, the Tabs
component would pass `selectedKey={null}` to React Aria's Tabs,
which triggers an infinite render loop.

This fix uses an empty string instead of null to indicate no
selection, which React Aria handles correctly.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.changeset/rare-papers-decide.md

@@ -0,0 +1,5 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed an infinite render loop in Tabs when navigating to a URL that doesn't match any tab href.

packages/ui/src/components/Tabs/Tabs.tsx

@@ -135,9 +135,10 @@ export const Tabs = (props: TabsProps) => {
       return undefined;
     }
 
-    // Has routed tabs but none are active - controlled mode with no selection
+    // Has routed tabs but none are active - use empty string for no selection
+    // (React Aria has a bug with null that causes infinite loops)
     if (activeTabs.size === 0) {
-      return null;
+      return '';
     }
 
     let selectedId: string | null = null;