Support nil activestorage objects

[EdwardIII]

Fixes "Module::DelegationError: size delegated to attachment, but
attachment is nil"

[coveralls]

Coverage increased (+33.8%) to 98.764% when pulling c140b79 on adaodesign:support_empty_activestorage_files into 00cb914 on musaffa:master.

[musaffa]

1. Need to fix the failing tests. Module::DelegationError can be replaced with a generic error.
2. A similar change is needed in the file content type validator.