Fix copy-paste error in hash_record_extended()

Fix copy-paste error in hash_record_extended()

The code failed to initialize the second isnull argument passed to
FunctionCallInvoke().  This is harmless for existing in-core extended
hash support functions, since FunctionCallInvoke() does not use the
value (note that all the in-core extended hash functions are strict),
examining only the argument values.  However, extension-provided
extended hash functions could be affected if they inspect
PG_ARGISNULL(1).

Oversight in 01e658fa74cb.

Author: Man Zeng <zengman@halodbtech.com>
Discussion: https://postgr.es/m/tencent_7818173C01E01836109848C3@qq.com
Backpatch-through: 14