Fix ncurses fallback.c build with a strict build shell

The script uses shift three times and when building with a strict /bin/sh
shifting without any arguments will cause the script to fail. In this case
the target will fail and we write an empty output file. When doing a
NO_CLEAN build after this will mean fallback.c is up to date and clang
will happily compile the empty input file which leads to strange build
errors later.

Fixed by passing three empty arguments to MkFallback.sh and only creating
fallback.c if MKfallback.sh succeeds.

Aproved By: brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D16867