Let User::idFromName always return int or null

Let User::idFromName always return int or null

This patch makes sure, that idFromName always returns either an int
or null value. The $idCacheByName can contain string values so we cast
the values if necessary.

An alternative could have been to make sure that just int values go
into the cache. But since the cache is public the current approach
to seems to be better atm.

Change-Id: I3085d89b93db2b888c190ba193623b86dc93759a