mailmap: debug: avoid passing NULL to fprintf() '%s' conversion specification

mailmap: debug: avoid passing NULL to fprintf() '%s' conversion specification

POSIX does not state the behavior of '%s' conversion when passed a
NULL pointer. Some implementations interpolate literal "(null)";
others may crash.

Callers of debug_mm() often pass NULL as indication of either a
missing name or email address.  Instead, let's always supply a
proper string pointer, and make it a bit more descriptive: "(none)"

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>