CMakeManager: return early if compilation data is invalid

Invalid compilation data has empty files and fileForFolder containers,
so CMakeManager::fileInformation() returns an empty CMakeFile anyway in
the end. Return early to prevent debug message spam when opening a CMake
project fails (e.g. due to a mistake in CMakeLists.txt) while
kdevelop.plugins.cmake.debug=true. Without the early return, the
    kdevelop.plugins.cmake: no information found for "/path/to/file"
message is printed 6 times for each project file in this scenario.