Use XLogRecPtrIsValid() in various places

Now that commit 06edbed478 has introduced XLogRecPtrIsValid(), we can
use that instead of:

- XLogRecPtrIsInvalid()
- direct comparisons with InvalidXLogRecPtr
- direct comparisons with literal 0

This makes the code more consistent.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/aQB7EvGqrbZXrMlg@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/access/heap/vacuumlazy.c

@@ -1901,7 +1901,7 @@ lazy_scan_new_or_empty(LVRelState *vacrel, Buffer buf, BlockNumber blkno,
 			 * WAL-logged, and if not, do that now.
 			 */
 			if (RelationNeedsWAL(vacrel->rel) &&
-				PageGetLSN(page) == InvalidXLogRecPtr)
+				!XLogRecPtrIsValid(PageGetLSN(page)))
 				log_newpage_buffer(buf, true);
 
 			PageSetAllVisible(page);