Found while confirming MBS-12917.

The Preview section in the relationship editor wrongly shows the previously used credits for target even though it isn't creditable.

Steps to reproduce:

1. Edit an artist
2. Add a relationship
3. Select a target type that supports credits such as Artist
4. Select a random relationship type and target
5. Set a random string for "Credited as"
   At this point, the preview correctly shows the credits for the relationship
6. Switch the target type to something that doesn't support credits such as Series
   At this point, the "Credited as" field is correctly not visible
7. Select a random relationship type and target
   At this point, the relationship preview wrongly prints the previous credits (from step 5) instead of the name of the newly selected series.