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

cmake/OpenCVCompilerOptimizations.cmake

@@ -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")