#400 Post-Python 3: Merge Titles can leave Content data when merging an OMNIBUS with another title type

[Ahasuerus]

Merge Titles leaves Content Indicator data when merging an OMNIBUS with another title type and selecting that other title type as the merged one. The logic should set the content indicator field's value to NULL because only OMNIBUS titles can have Content Indicator data. See the "story length" merge logic for an example of how it can be done.