Fix calculation of maximal multixact in ingest_multixact_create_record (#6502)

## Problem

See https://neondb.slack.com/archives/C06F5UJH601/p1706373716661439

## Summary of changes

Use None instead of 0 as initial accumulator value for calculating
maximal multixact XID.

## 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>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

test_runner/regress/test_next_xid.py

@@ -203,6 +203,16 @@ def test_import_at_2bil(
         $$;
         """
     )
+
+    # Also create a multi-XID with members past the 2 billion mark
+    conn2 = endpoint.connect()
+    cur2 = conn2.cursor()
+    cur.execute("INSERT INTO t VALUES ('x')")
+    cur.execute("BEGIN; select * from t WHERE t = 'x' FOR SHARE;")
+    cur2.execute("BEGIN; select * from t WHERE t = 'x' FOR SHARE;")
+    cur.execute("COMMIT")
+    cur2.execute("COMMIT")
+
     # A checkpoint writes a WAL record with xl_xid=0. Many other WAL
     # records would have the same effect.
     cur.execute("checkpoint")
@@ -217,4 +227,4 @@ def test_import_at_2bil(
     conn = endpoint.connect()
     cur = conn.cursor()
     cur.execute("SELECT count(*) from t")
-    assert cur.fetchone() == (10000 + 1,)
+    assert cur.fetchone() == (10000 + 1 + 1,)