fix(email-service) Handle null attachment filename during upload

[evanhutnik]

Summary

There is an edge case where the file name for an attachment can be null. Updating the attachment upload logic to handle this. We now return an error if the caller attempts to upload a DSS attachment with a null filename (name in Document table can't be null), whereas we upload image/video attachments with null filenames to SFS still (we don't care about the file name when uploading static media). The null edge case only appears to occur with image files anyway (how would you have a nameless pdf? not really possible)

Adding tests to validate the queries that fetch documents to upload to DSS don't return attachments with null filenames.

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