ePub: don't always expect submissions types from content export

fixes CNVS-25459

test plan:
- ePub export regression tests

Change-Id: I20c47e086c8141ba5165e32bfd4bdfec113f629c
Reviewed-on: https://gerrit.instructure.com/68026
Reviewed-by: Matt Berns <mberns@instructure.com>
Tested-by: Jenkins
Product-Review: John Corrigan <jcorrigan@instructure.com>
QA-Review: John Corrigan <jcorrigan@instructure.com>

lib/cc/exporter/epub/converters/assignment_epub_converter.rb

@@ -48,7 +48,7 @@ module CC::Exporter::Epub::Converters
       assignment
     end
 
-    def submission_types(types)
+    def submission_types(types="")
       types.split(",").map{|sub_type| CartridgeConverter::SUBMISSION_TYPES[sub_type]}
     end
   end