Fix missing initialization in pg_restore_extended_stats()

Fix missing initialization in pg_restore_extended_stats()

The tuple data upserted into pg_statistic_ext_data was missing an
initialization for the nulls flag of stxoid and stxdinherit.  This would
cause an incorrect handling of the stats data restored.

This issue has been spotted by CatalogTupleCheckConstraints(),
translating to a NOT NULL constraint inconsistency, while playing more
with the follow-up portions of the patch set.

Oversight in 0e80f3f88dea (mea culpa).  Surprisingly, the buildfarm did
not complain yet.

Discussion: https://postgr.es/m/CADkLM=c7DY3Jv6ef0n_MGUJ1FyTMUoT697LbkST05nraVGNHYg@mail.gmail.com