Avoid crash in "postgres -C guc" for a GUC with a null string value.

Emit "(null)" instead, which was the behavior all along on platforms
that don't crash, eg OS X.  Per report from Jehan-Guillaume de Rorthais.
Back-patch to 9.2 where -C option was introduced.

Michael Paquier

Report: <20160615204036.2d35d86a@firost>