Why clearing the VM doesn't require registering vm buffer in wal record

[Melanie Plageman]

Today Andres and I noticed that heap_{update,insert,delete}() don't
register the VM buffer when they are clearing the VM. I was under the
impression that any buffers modified needed to be registered in the
WAL record. Without which, you'll never do an FPI. It seems like this
could cause checksum failures. What are we missing?