Do not assign page LSN to new (uninitialized) page in ClearVisibilityMapFlags redo handler (#9287)

## Problem

https://neondb.slack.com/archives/C04DGM6SMTM/p1727872045252899

See https://github.com/neondatabase/neon/issues/9240

## Summary of changes

Add `!page_is_new` check before assigning page lsn.

## 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/walredo/apply_neon.rs

@@ -67,7 +67,10 @@ pub(crate) fn apply_in_neon(
                 let map = &mut page[pg_constants::MAXALIGN_SIZE_OF_PAGE_HEADER_DATA..];
 
                 map[map_byte as usize] &= !(flags << map_offset);
-                postgres_ffi::page_set_lsn(page, lsn);
+                // The page should never be empty, but we're checking it anyway as a precaution, so that if it is empty for some reason anyway, we don't make matters worse by setting the LSN on it.
+                if !postgres_ffi::page_is_new(page) {
+                    postgres_ffi::page_set_lsn(page, lsn);
+                }
             }
 
             // Repeat for 'old_heap_blkno', if any
@@ -81,7 +84,10 @@ pub(crate) fn apply_in_neon(
                 let map = &mut page[pg_constants::MAXALIGN_SIZE_OF_PAGE_HEADER_DATA..];
 
                 map[map_byte as usize] &= !(flags << map_offset);
-                postgres_ffi::page_set_lsn(page, lsn);
+                // The page should never be empty, but we're checking it anyway as a precaution, so that if it is empty for some reason anyway, we don't make matters worse by setting the LSN on it.
+                if !postgres_ffi::page_is_new(page) {
+                    postgres_ffi::page_set_lsn(page, lsn);
+                }
             }
         }
         // Non-relational WAL records are handled here, with custom code that has the