core-app-api: fix index route gathering

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/cuddly-bikes-tease.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core-app-api': patch
+---
+
+Fixed a bug where gathered index routes would fail to bind routable extensions. This would typically show up when placing a routable extension in the entity page overview tab.

packages/core-app-api/src/routing/collectors.stable.test.tsx

@@ -301,7 +301,7 @@ describe('discovery', () => {
             HELLO
           </AggregationComponent>
           <Route path="bar" element={<Extension3 />}>
-            <AggregationComponent path="baz">
+            <AggregationComponent path="">
               <Extension4>
                 <Extension5 />
               </Extension4>
@@ -322,8 +322,8 @@ describe('discovery', () => {
       [ref1, 'foo'],
       [ref2, 'foo'],
       [ref3, 'bar'],
-      [ref4, 'baz'],
-      [ref5, 'baz'],
+      [ref4, ''],
+      [ref5, ''],
     ]);
     expect(sortedEntries(routing.parents)).toEqual([
       [ref1, undefined],
@@ -337,7 +337,7 @@ describe('discovery', () => {
       routeObj(
         'bar',
         [ref3],
-        [routeObj('baz', [ref4, ref5], [], 'gathered')],
+        [routeObj('', [ref4, ref5], [], 'gathered')],
         undefined,
         plugin,
       ),

packages/core-app-api/src/routing/collectors.tsx

@@ -178,7 +178,7 @@ export const routingV2Collector = createCollector(
     }
 
     if (mountPoint) {
-      if (!ctx?.gatherPath) {
+      if (ctx?.gatherPath === undefined) {
         throw new Error(
           `Routable extension "${stringifyNode(
             node,