pgsql: Remove partial, broken support for NULL pointers when fetching

Log Message:
-----------
Remove partial, broken support for NULL pointers when fetching attributes.

Previously, fastgetattr() and heap_getattr() tested their fourth argument
against a null pointer, but any attempt to use them with a literal-NULL
fourth argument evaluated to *(void *)0, resulting in a compiler error.
Remove these NULL tests to avoid leading future readers of this code to
believe that this has a chance of working.  Also clean up related legacy
code in nocachegetattr(), heap_getsysattr(), and nocache_index_getattr().

The new coding standard is that any code which calls a getattr-type
function or macro which takes an isnull argument MUST pass a valid
boolean pointer.  Per discussion with Bruce Momjian, Tom Lane, Alvaro
Herrera.

Modified Files:
--------------
    pgsql/src/backend/access/common:
        heaptuple.c (r1.129 -> r1.130)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/heaptuple.c?r1=1.129&r2=1.130)
        indextuple.c (r1.90 -> r1.91)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/indextuple.c?r1=1.90&r2=1.91)
    pgsql/src/backend/access/heap:
        heapam.c (r1.280 -> r1.281)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.280&r2=1.281)
    pgsql/src/include/access:
        htup.h (r1.109 -> r1.110)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/htup.h?r1=1.109&r2=1.110)
        itup.h (r1.53 -> r1.54)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/itup.h?r1=1.53&r2=1.54)