Change various deparsing functions to return NULL for invalid input.

Previously, some functions returned various fixed strings and others
failed with a cache lookup error.  Per discussion, standardize on
returning NULL.  Although user-exposed "cache lookup failed" error
messages might normally qualify for bug-fix treatment, no back-patch;
the risk of breaking user code which is accustomed to the current
behavior seems too high.

Michael Paquier

Author: robertmhaas