Optimize storing of null page in WAL (#5910)

## Problem

PG16 (neondatabase/postgres#327) adds new
function to SMGR: zeroextend
It's implementation in Neon actually wal-log zero pages of extended
relation.
This zero page is wal-logged using XLOG_FPI.
As far as page is zero, the hole optimization (excluding from the image
everything between pg_upper and pd_lower) doesn't work.

## Summary of changes

In case of zero page (`PageIsNull()` returns true) assume
`hole_size=BLCKSZ`

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <[email protected]>

vendor/revisions.json

@@ -1,5 +1,5 @@
 {
-    "postgres-v16": "763000f1d0873b827829c41f2f6f799ffc0de55c",
+    "postgres-v16": "e3a22b72922055f9212eca12700190f118578362",
     "postgres-v15": "bc88f539312fcc4bb292ce94ae9db09ab6656e8a",
     "postgres-v14": "dd067cf656f6810a25aca6025633d32d02c5085a"
 }