opening_user_name can be nil

[peetucket]

Why was this change made? 🤔

We allow opening_user_name to be nil when opening a new version, but it is possible to get an HB exception if you leave it off when calling the accession method from DSA cient, as shown below. This should fix the problem by allowing it to be nil via this pathway too. Error below, triggers this HB alert: https://app.honeybadger.io/projects/50568/faults/107835795 Adding the opening_user_name let's it work:

object_client = Dor::Services::Client.object('druid:zb460rp4078')
object_client.accession.start(description: 'do it!', context: {'runOCR' => true})

/Users/petucket/Sites/development/dor-services-client/lib/dor/services/client/versioned_service.rb:45:in `raise_exception_based_on_response!': Internal Server Error: 500 (Response from dor-services-app did not contain a body.                         Check honeybadger for dor-services-app for backtraces,                         and look into adding a `rescue_from` in dor-services-app                         to provide more details to the client in the future) (Dor::Services::Client::UnexpectedResponse)
    from /Users/petucket/Sites/development/dor-services-client/lib/dor/services/client/accession.rb:34:in `start'
    from (irb):34:in `<main>'
    from <internal:kernel>:187:in `loop'
    from bin/console:15:in `<main>'

object_client.accession.start(opening_user_name: 'petucket', description: 'do it!', context: {'runOCR' => true})
 => true

The reason is that most consumers just call VersionService.open, which sets the opening_user_name to nil if left off. However, DSA's client call results in a VersionService.new and then later calls #open on the instance of the object (see https://github.com/sul-dlss/dor-services-client/blob/main/lib/dor/services/client/accession.rb#L16-L31 and the resulting call ends up at https://github.com/sul-dlss/dor-services-app/blob/main/app/controllers/objects_controller.rb#L72-L100), passing whatever params the consumer sent to DSA client. If the user doesn't send opening_user_name, it will not be sent to #open at all, and since that method is not defining a default, it blows up. This just lets the default be nil, as it does in the .open method.

How was this change tested? 🤨

⚡ ⚠ If this change has cross service impact, including data writes to shared file systems, run integration tests and/or test in [stage|qa] environment, in addition to specs. ⚡