Do not materialize null images in PS (#5979)

## Problem

PG16 is writing null images during relation extension.
And page server implements optimisation which replace WAL record with
FPI with page image.
So instead of WAL record ~30 bytes we store 8kb null page image.
Ans this image is almost useless, because most likely it will be shortly
rewritten with actual page content.

## Summary of changes

Do not materialize wal records with null page FPI.
 
## 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 <knizhnik@neon.tech>

pageserver/src/walingest.rs

@@ -458,8 +458,10 @@ impl<'a> WalIngest<'a> {
             && decoded.xl_rmid == pg_constants::RM_XLOG_ID
             && (decoded.xl_info == pg_constants::XLOG_FPI
                 || decoded.xl_info == pg_constants::XLOG_FPI_FOR_HINT)
-        // compression of WAL is not yet supported: fall back to storing the original WAL record
+            // compression of WAL is not yet supported: fall back to storing the original WAL record
             && !postgres_ffi::bkpimage_is_compressed(blk.bimg_info, self.timeline.pg_version)?
+            // do not materialize null pages because them most likely be soon replaced with real data
+            && blk.bimg_len != 0
         {
             // Extract page image from FPI record
             let img_len = blk.bimg_len as usize;