CMake: Return an empty project tree if parsing failed

This triggers the logic in Project to show the top level project
file in the project tree.

Remove similar logic in CMake.

Change-Id: I2bfdd3f5e3d4126910a3feb480cec5ef689954ed
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Florian Apolloner <florian@apolloner.eu>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.cpp

@@ -221,6 +221,10 @@ CMakeBuildConfiguration::generateProjectTree(const QList<const FileNode*> &allFi
 
     auto root = new CMakeProjectNode(target()->project()->projectDirectory());
     m_buildDirManager->generateProjectTree(root, allFiles);
+    if (root->isEmpty()) {
+        delete root;
+        return nullptr;
+    }
     return root;
 }