cmake: reset __content variable if file doesn't exist
cmake: reset __content variable if file doesn't exist

Resolves CMake error after relaunch with updated source code:
Cannot find source file:
    modules/dnn/layers/layers_common.avx512_skx.cpp

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv