awk: Make -F '' and -v FS="" behave the same

IEEE Std 1003.1-2008 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="".

PR: 241441
Upstream issue: https://github.com/onetrueawk/awk/issues/127
Upstream pull request: https://github.com/onetrueawk/awk/pull/128
MFC After: 2 weeks
Sponsored by: Netflix