Fix bug with AS_IF and ‘set -e’

Fix bug with AS_IF and ‘set -e’

Problem reported by Peter Johansson in:
https://lists.gnu.org/r/bug-autoconf/2022-03/msg00009.html
The bug was introduced in 2020-11-15T18:56:18Z!zackw@panix.com
with commit message saying “If anyone has a better idea for how to
make this work I will be glad to hear it”, and this patch uses a
trick with ‘case’ that should be a better idea.
* lib/m4sugar/m4sh.m4 (as_nop, as_fn_nop, _AS_EMPTY_ELSE_PREPARE):
Remove.  All uses removed.
(_AS_IF_ELSE): Wrap the non-blank IF-ELSE inside a no-op case,
which works since a case branch can be empty in the shell.
* tests/m4sh.at (AS_IF and AS_CASE): Test the fix.