Guard against nil tempfiles in MARC export job

Why these changes are being introduced:

If a tempfile has already been closed and unlinked, the `ensure`
block in MarcExportJob#perform will throw an error.

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/ETD-546

How this addresses that need:

This checks if the tempfile exists before attempting to close it.

Side effects of this change:

None.

app/jobs/marc_export_job.rb

@@ -8,7 +8,7 @@ def perform(theses)
       zip_file = MarcBatch.new(theses, marc_filename, zip_filename).build
       BatchMailer.marc_batch_email(zip_filename, zip_file, theses).deliver_now
     ensure
-      zip_file.close
+      zip_file.close if zip_file
     end
   end