check is_array before implode

I realize that no one else is probably having this issue, but I don't see the harm in it either. The XML Importer never checks if the value is an array before it attempts to implode it if multiple authors aren't allowed. I know it is in the weeds and if everything were working perfectly you would never have to worry about a string coming in a causing this error, but checking if it is an array first is the difference between me being able to run my importer successfully and it failing with an error.