config: really treat missing optional path as not configured

config: really treat missing optional path as not configured

These callers expect that git_config_pathname() that returns 0 is a
signal that the variable they passed has a string they need to act
on.  But with the introduction of ":(optional)path" earlier, that is
no longer the case.  If the path specified by the configuration
variable is missing, their variable will get a NULL in it, and they
need to act on it (often, just refraining from copying it elsewhere).

Signed-off-by: Junio C Hamano <gitster@pobox.com>