export_as_mla_citation_txt encounters a nilClass on production

[regineheberlein]

Expected behavior

A record can be exported in MLA format

Actual behavior

It serves the "Looks like something went wrong!" error page and reports
NoMethodError: undefined method `+' for nil:NilClass return temp_name.last + " " + temp_name.first

Steps to replicate

https://catalog.princeton.edu/catalog/99126151309006421/citation

Impact of this bug

Patrons cannot export citations in MLA format

Honeybadger link and code snippet, if applicable

Honeybadger report

NoMethodError: undefined method `+' for nil:NilClass return temp_name.last + " " + temp_name.first ^
    return name unless name =~ /,/
    temp_name = name.split(", ")
    return temp_name.last + " " + temp_name.first
  end
end```

### Implementation notes, if any

[sandbergja]

I believe this happens because the record has a 700$a subfield with the content , . (comma space period).

The fix would likely be in this file in the blacklight-marc gem, with a test perhaps in this file

[sandbergja]

This will be resolved upstream by projectblacklight/blacklight-marc#117

[sandbergja]

The fix is now in blacklight-marc 8.1.2, we just need to upgrade and test to resolve this issue.