Avoid returning `[nil]` to Rack

Prior to Rack v2.1.0, Rack::Response would call to_s on each of the body parts: https://github.com/rack/rack/blob/85684323f8f58409e717af91e446d257d496f8b8/lib/rack/response.rb#L40-L43. That means if Grape returned [nil], Rack would handle this gracefully.

This is no longer the case now with https://github.com/rack/rack/pull/1434/files. It's easy to have an API call that returns nil to Grape, and the body will be set to [nil]: https://github.com/ruby-grape/grape/blob/0d5cf4035c04c1c44e7868a8b77a969b28766245/lib/grape/endpoint.rb#L267-L277

This causes fits with Rack:

NoMethodError (undefined method `empty?' for nil:NilClass):
rack (2.1.4) lib/rack/etag.rb:70:in `block in digest_body'
rack (2.1.4) lib/rack/body_proxy.rb:34:in `block in each'
rack (2.1.4) lib/rack/body_proxy.rb:34:in `each'
rack (2.1.4) lib/rack/body_proxy.rb:34:in `each'
rack (2.1.4) lib/rack/etag.rb:68:in `digest_body'
rack (2.1.4) lib/rack/etag.rb:31:in `call'
rack (2.1.4) lib/rack/conditional_get.rb:40:in `call'
rack (2.1.4) lib/rack/head.rb:14:in `call'

Maybe Grape should return nil instead of [nil] if there is no body? This might have to be done in the formatters since nil may mean null in JSON.

Rack issue: https://github.com/rack/rack/issues/1712

Start in lib/grape/endpoint.rb at the body assignment around lines 267-277, then inspect the formatters mentioned in the issue and Rack's changed response handling. Reproduce an endpoint returning nil with Rack 2.1.4 and determine whether an empty body must be returned differently from a JSON null value. Add or update coverage for both cases if the relevant test location is identified.