message: 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
diff: @@ -760,6 +760,8 @@ macro(ocv_add_dispatched_file filename)
  set(__file "${CMAKE_CURRENT_BINARY_DIR}/${filename}.${OPT_LOWER}.cpp")
  if(EXISTS "${__file}")
  file(READ "${__file}" __content)
  + else()
  + set(__content "")
  endif()
  if(__content STREQUAL __codestr)
  #message(STATUS "${__file} contains up-to-date content")
  