Fix #4238 - if should match undef as ''

The comparisons operator (and more particularly == and !=) were not treating
the undef value as '', like case and selector did since #2818.

This patch makes sure comparison operator uses AST leaf matching.

Unfortunately, doing this introduces a behavior change compared to
the previous versions:
Numbers embedded in strings will now be matched as numbers in case and
selector statements instead of string matching.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>