BUG Calling DataObject::relField() on a object with an empty relation list

This causes a 'Fatal error: Call to a member function hasMethod() on a non-object'.

This can happen when displaying a field in a gridfield on a belongs_to relationship.
Merge pull request #1055 from stojg/bug-relField

BUG Calling DataObject::relField() on a object with an empty relation list