Fix incorrect presorted DISTINCT aggregate if condition

Here we fix a faulty "if" condition which failed to correctly handle two
or more consecutive NULL transition values when checking if the new value
is DISTINCT from the old value for presorted aggregates.  Given a suitably
non-strict aggregate transition function, a byref aggregate could cause a
crash due to calling the type's equality function and passing along a
(Datum) 0 value to test for equality, the equality function would then try
to dereference that 0 Datum and segfault.  For byval types, there'd have
been no crash and the equality function would have seen that the two 0
Datums matched, which (only by chance) meant the calling code would have
worked correctly.

Here we ensure that we only call the equality function when neither of
the input values are NULL.

This code is all new as of 1349d2790, so no backpatch needed.

Reported-by: Fujii Masao
Discussion: https://postgr.es/m/860c6d6f-a3c5-3ae9-9da2-827177bede06@oss.nttdata.com

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