Reshape::FieldsToFieldGroupWithConstant adds constant when no field group values

Tried in migration project: 

```
            transform Reshape::FieldsToFieldGroupWithConstant,
              fieldmap: {
                ben_assocstructureddategroup: :assocstructureddategroup
              },
              constant_target: :assocdatetype,
              constant_value: 'received'
```

Added "received" to all rows, even those with no assocstructureddategroup values