Make "postgres -C guc" print "" not "(null)" for null-valued GUCs.

Commit 0b0baf26 et al made this case print "(null)" on the grounds that
that's what happened on platforms that didn't crash.  But neither behavior
was actually intentional.  What we should print is just an empty string,
for compatibility with the behavior of SHOW and other ways of examining
string GUCs.  Those code paths don't distinguish NULL from empty strings,
so we should not here either.  Per gripe from Alain Radix.

Like the previous patch, back-patch to 9.2 where -C option was introduced.

Discussion: <CA+YdpwxPUADrmxSD7+Td=uOshMB1KkDN7G7cf+FGmNjjxMhjbw@mail.gmail.com>