(bug 62634) Fatal exception "DatabaseBase::makeList: empty input for field rev_id" on Special:Contributions

$id was a UUID object here. Using it as array key results in warning
"Illegal offset type", causing the element not to be added to $pks,
resulting in an empty array being fed to findRevIdReal() which, in
turn, will attempt to build a query "WHERE 'rev_id' = <empty>",
resulting in an error from DatabaseBase::makeList.

This likely only happened on Special:Contributions, since it's
one of the few places where we can't read data from cache.
It happened inconsistently, because it only attempts this code
path when there are 3 different "queries". In the case of this
specific error: this happened when a user has > 3 Flow entries in
his/her contributions, that had previous revisions. So this mainly
occurs for people editing/moderating a lot in Flow.

Bug: 62634
Change-Id: I9ed1486cd203a324e24c1f62d0f38d2e0d95cc30