[2] RightsStatement field is not required and not inherited if empty

[rschwab]

Summary As an admin user, I want to be able to ingest works without a RightsStatement URI. Some works are intentionally left without this field, and so empty values should be allowed and not automatically inherit a default value from the importer.

Acceptance Criteria

• [x] Works can be imported a blank RightsStatement field
• [x] Spreadsheets can be imported without a RightsStatement column
• [x] When RightsStatement is not declared on an import, it goes into the repository with an empty value and is not inherited from the importer default, the collection, or a parent work.
• [x] When records with blank rightsStatements are exported, they are not assigned a rightsStatement

[bkiahstroud]

We decided that our usage of rightsStatement and rightsStatus would be switched: the rightsStatements.org values will now go in rightsStatement, and the descriptive text will go in rightsStatus.

- Rachel (Slack)