Fix motion toast error.

[gfphoenix78]

slot->tts_isnull cannot be read before invoking slot_getallattrs.

Fix Github Issue 16906.

Authored-by: wenxing.yaun wenxing.yuan@esgyn.cn

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[gfphoenix78]

The issue was inherited from greenplum, see https://github.com/greenplum-db/gpdb/issues/16906
It had been fixed in https://github.com/greenplum-db/gpdb/pull/16907
This PR is simply a cherry-pick.