ResponseMethods#to_formatted_s incorrectly assumes non-nil datetime_value

See https://github.com/NUBIC/surveyor/blob/master/lib/surveyor/models/response_methods.rb#L101.

Also see https://github.com/NUBIC/surveyor/blob/v1.4.0/lib/surveyor/models/response_methods.rb#L91, which [affects NCS applications](https://code.bioinformatics.northwestern.edu/issues/issues/show/4389).