Merge pull request opencv#10845 from alalek:cpu_dispatch_fix_file_generation

Author: alalek

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")