CoreParserFunctions: Return 0 from {{PAGESIZE:}} when length is unknown

CoreParserFunctions: Return 0 from {{PAGESIZE:}} when length is unknown

Revision::getSize() might return null when the revision.rev_len field
is null. That should never happen normally (the field should get
backfilled as part of the update process), but we've also had a bug
where rev_len was not being recorded for empty pages (see T135414 for
details). It's saner to return a number here rather than empty string,
and 0 should actually be correct for all pages affected by that issue.

Bug: T20998
Change-Id: Ie12f0be24f00aaf8b90b25c4921a97df3b789369