Issue #3154084 by amateescu, mheip, smustgrave: Warning when building an empty workspace tree

(cherry picked from commit 2a17c29f)

core/modules/workspaces/src/WorkspaceRepository.php

@@ -116,6 +116,7 @@ public function loadTree() {
       }
       $graph = (new Graph($graph))->searchAndSort();
 
+      $this->tree = [];
       foreach (array_keys($tree) as $workspace_id) {
         $this->tree[$workspace_id] = [
           'depth' => count($graph[$workspace_id]['paths']),

core/modules/workspaces/tests/src/Kernel/WorkspaceCRUDTest.php

@@ -323,4 +323,12 @@ public function testDeletingWorkspaceWithChildren() {
     $this->assertNull(Workspace::load('stage'));
   }
 
+  /**
+   * Tests loading the workspace tree when there are no workspaces available.
+   */
+  public function testEmptyWorkspaceTree() {
+    $tree = \Drupal::service('workspaces.repository')->loadTree();
+    $this->assertSame([], $tree);
+  }
+
 }