Don't fail on incomplete row info when exporting.

Don't fail on incomplete row info when exporting.

This allows XmlDumpWriter to output rows that are missing some
information without triggering php warnings.

This fixes warnings caused by the OAI extension when using
XmlDumpWriter::writeRevision without setting the row's
rev_parent_id field.

Generally, it seems convenient to allow optional fields to be unset
instead of skipping them if they are empty, but requiering them to be
set.

Change-Id: I246ba3a899bd9e833232444a11ac63e1b0225ab6