WAL zero blocks should be encryption aware

When backend creates a new WAL segment file it usually fills it with ‘\0'. Currently it bypasses the WAL encryption. It worked well when we kept “what to encrypt“ info in page headers. But now, with ranges, all these zero writes should go through the tde WAL writer so they would be encrypted if needed. But then whet to do if in the middle of the segment wal_enc was turned off and we have legit unencrypted data followed by encrypted zeros? So maybe leave zeros alone and make the reader aware of it (don’t touch zero pages even if there is an encryption key)?