POSIX mandates that -F str be treated the same as -v FS=str.

POSIX mandates that -F str be treated the same as -v FS=str.
For a null string, this was not the case.  Since awk(1) documents
that a null string for FS has a specific behavior, make -F '' behave
consistently with -v FS="".  https://github.com/onetrueawk/awk/pull/128