CMake: Return empty tree on errors in server-mode

This triggers the fallback mechanism which will make sure the CMakeLists.txt
file will be displayed.

This used to fail since one empty folder was left over in the project.

Task-number: QTCREATORBUG-17383
Change-Id: I867fd9039bc5df805ff5174ec49084bbcf688824
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/cmakeprojectmanager/cmakeproject.h

@@ -113,7 +113,7 @@ private:
     void handleActiveTargetChanged();
     void handleActiveBuildConfigurationChanged();
     void handleParsingStarted();
-    void updateProjectData();
+    void updateProjectData(Internal::CMakeBuildConfiguration *cmakeBc);
     void updateQmlJSCodeModel();
 
     void createGeneratedCodeModelSupport();