Add Property::isNullable()
Add ScalarProperty::isNullable()
CollectionAssociationProperty::isNullable() returns always false
Implement ASsociationProperty::isNullable()
It needs to look into the JoinColumn metadata to find if
nullable=false is explicitly mentioned, then return false,
default return true.
Document return type of FormElement::getProperty()
Implement null checkboxes into form view script