[REF] Import - make condition clearer

[REF] Import - make condition clearer

The mapping of fields basically breaks down into 'has a relationship' - in which
case fieldName is empy and does not have a relationship - in which
case field name is not empy. This makes that explicit -
specifically referring to relatedContactKey rather than
relying on the person reading the code
understanding that the presence of fieldName implies
whether it is a relationship