ProjectExplorer: Use unique_ptr to hold ProjectNodes

Change-Id: Iaa5bea221686564de24138a99b5fe0d09521c118
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/plugins/cmakeprojectmanager/servermodereader.cpp

@@ -907,7 +907,7 @@ void ServerModeReader::addHeaderNodes(ProjectNode *root, const QList<FileNode *>
         }
     }
 
-    if (headerNode->nodes().isEmpty())
+    if (headerNode->isEmpty())
         delete headerNode; // No Headers, do not show this Folder.
     else
         root->addNode(headerNode);