When using the default Doctrine EntityType in Forms, invalid GUID/UUID values (e.g. "", "asdf", 1) are being run through the database, which causes exceptions when using PostgreSQL

If an invalid UUID is submitted in a form (including empty values, which ends up being sent to Postgres as "") an exception is thrown, e.g.

SQLSTATE[22P02]: Invalid text representation: 7 ERROR:  invalid input syntax for uuid: \"\"

The class already filters out non-int values for single int id entities for the same reason; because it causes an error on Postgres (see commit 45579fd). So it makes sense to me to run a similar check for valid UUID values.