Fix according to JSR-283 6.7.39

This PR started from https://github.com/phpcr/phpcr-api-tests/pull/108, doctrine-dbal returned an array.
The JSR-283 specs tell us that a query should only return single-valued properties. 

We discussed that for now we will use the same as jackrabbit does: return a concatenated string for string multi-valued properties.
But that only works for string properties, and because we can't create a good way to handle boolean/binary/date multi-valued properties, I choose to return `null` for those.

Because the method is defined @api this is a BC break.