avoid null pointer when getting email for OAI-PMH #3619

[pdurbin]

What this PR does / why we need it:

This pull request is a follow up to pull request #6513 where I accidentally introduced the chance of 500 errors being thrown at URLs such as http://phoenix.dataverse.org/oai?verb=Indentify when the system email address has not been configured:

Which issue(s) this PR closes:

Closes #3619 (re-closes)

Special notes for your reviewer:

None.

Suggestions on how to test this:

Configure Dataverse with and without a system email, stopping and starting Glassfish in between each configuration change. For both cases, go to a URL such as http://phoenix.dataverse.org/oai?verb=Indentify and make sure a 500 error is not thrown.

When the system email is not configured, you should see something like <adminEmail/> (I checked that this is how it works in 4.18.1 so it's the same.).

When the system email is configured, only the email address should be shown like this: <adminEmail>[email protected]</adminEmail>. As with the the original change in #6513 in order to help OAI-PMH validate, we should never see the "personal" part like `Dataverse Support [email protected].

Does this PR introduce a user interface change?:

No.

Is there a release notes update needed for this change?:

No.

Additional documentation:

None.

[coveralls]

Coverage decreased (-0.0003%) to 19.501% when pulling 024b8b2 on 3619-avoid-npe into 804eac3 on develop.