fix(posting): re-enable IsEmpty check in IterateDisk

The original branch commit 41d6445 ("fixed some bug") replaced
the IsEmpty(readTs) call in IterateDisk with a hardcoded `false`,
forcing every key found by the iterator to be reported as non-empty.
That broke has(<predicate>) for any uid whose value had been removed
via star-deletion (<uid> <pred> *): the data list still exists in
badger with a DeleteAll marker on top, but the live posting list is
empty at readTs — IsEmpty returns true and the uid should be skipped.

Surfaced by systest TestSystestSuite/TestHasDeletedEdge in
systest/mutations-and-queries: 3 nodes are created with <end> "",
one is star-deleted, follow-up has(end) is expected to return 2 uids.
With IsEmpty stubbed to false it returned 3.

No comment was left on the original change. Restoring the call. The
mutations-and-queries package is fully green with this in place
(66/66 tests pass including TestHasDeletedEdge); if a real underlying
issue motivated the original disable we'll chase it with a real
diagnosis instead of silently dropping a safety check.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: matthewmcneely

posting/mvcc.go

@@ -496,8 +496,7 @@ func (ml *MemoryLayer) IterateDisk(ctx context.Context, f IterateDiskArgs) error
 		if err != nil {
 			return err
 		}
-		empty, err := false, nil
-		//empty, err := l.IsEmpty(f.ReadTs, 0)
+		empty, err := l.IsEmpty(f.ReadTs, 0)
 		switch {
 		case err != nil:
 			return err