Allow nil to be passed to get_email_override_settings
Allow nil to be passed to get_email_override_settings

- Since this method is used internally, it should always be sent
  something, even if it is nil. This will allow the setting to be
  skipped, and return nothing.
- A minor bug that was preventing the envelope from succeeding if
  a recipient_id is not passed with the signer (such as in the
  test). This allows the test to pass, since DocuSign is not
  expecting an empty string.

kcbigring/docusign_rest