pgsql: Fix amcheck for page checks concurrent to replay of btree page d

Fix amcheck for page checks concurrent to replay of btree page deletion

amcheck expects at least hikey to always exist on leaf page even if it is
deleted page.  But replica reinitializes page during replay of page deletion,
causing deleted page to have no items.  Thus, replay of page deletion can
cause an error in concurrent amcheck run.

This commit relaxes amcheck expectation making it tolerate deleted page with
no items.

Reported-by: Konstantin Knizhnik
Discussion: https://postgr.es/m/CAPpHfdt_OTyQpXaPJcWzV2N-LNeNJseNB-K_A66qG%3DL518VTFw%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Peter Geoghegan
Backpatch-through: 11

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ae1f9b0a9bc3d2d1e46fcd38c8b2e7ba49238b05

Modified Files
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contrib/amcheck/verify_nbtree.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)