Fix relative path edge case in ECMGeneratePriFile

When the qt install prefix and the qt host data path are the same, CMake's RELATIVE_PATH file path function will return the empty string. This made us accidentally set ECM_MKSPECS_INSTALL_DIR to /mkspecs/modules. Fix this by explicitly checking for the empty string. Once we can depend on CMake 3.20, we can use CMake's builtin function for joining paths instead.