Skip empty custom fields during de-dupe

Partly related to CRM-6235

Empty values in custom fields are causing contacts to be flagged as duplicates.

Steps to recreate
Add a custom field to a contact and use a general dedupe rule that uses the field. All contacts will be marked as duplicates because no contacts have a value in the field.

Possible solution
This could be fixed by adding something like:
field_name <> ''
into the where statements when de-duping, but opening this up for further discussion.