… errors

initialize() is called sometimes, even when the following code doesn't need
the targetPlatform property. Specifically, in AbstractClassMetadataFactory::getAllMetadata().

But as of DBAL 2.5.0, calling Connection::getDatabasePlatform() will make a
connection to the database, which means that sometimes it may fail (e.g. you
haven't configured your database yet). As a result, calling a method like
AbstractClassMetadataFactory::getAllMetadata() - which does not need the
targetPlatform - will fail, because determining the targetPlatform requires
a connection, which fails.

This avoids that - we only get the targetPlatform *when* we need it, which
are cases where we're doing things that do indeed need a connection.