add missing mock to snapshot cache test

Author: JKobrynski

tests/unit/OnyxSnapshotCacheTest.ts

@@ -158,6 +158,7 @@ describe('OnyxSnapshotCache', () => {
 
         it('should invalidate non-collection keys without affecting others', () => {
             mockedOnyxUtils.isCollectionKey.mockReturnValue(false);
+            mockedOnyxUtils.getCollectionKey.mockReturnValue(null);
 
             cache.invalidateForKey('nonCollectionKey');