Optionally consider empty values when evaluating uniqueness

The issue #225887: Null field is not unique → introduced a "fix" which meant you could not compare empty values. However there are a number of use cases where this might be desirable.

For example, say you have a database of people with uniqueness based on names and telephone number. On the one phone number there's John Smith, and his son John Smith Jr. If you have 'field_first_name', 'field_last_name', 'field_name_suffix', and 'field_phone' determining uniqueness, John Smith Jr. will not be considered the same person as John Smith, but John Smith will never be considered the same person as John Smith because the current code says "nope, no value for field_name_suffix, nothing can match".

So in some cases, considering empty values is desirable, in others not. The attached patch makes this an option on a per-content-type basis.