Option expose_nil doesn't work when block is passed

[serbiant]

Greetings,

I've been trying to implement a conditional exposure at one of the entities in my project.
Noticed that in the current implementation, this code

expose :personal_details, expose_nil: false do |address, options|
  if options[:one_option].present?
    Public::Entities::PersonalDetail.represent address.personal_details
  elsif options[another_option].present?
    Public::Entities::PersonalDetailShort.represent address.personal_details
  else
    nil # explicit for the example
  end
end

will eventually expose null if both conditions are not satisfied.

This PR fixes the specified issue.

[coveralls]

Coverage increased (+0.01%) to 95.869% when pulling 3e2499f on serbiant:master into 54f68b2 on ruby-grape:master.

[LeFnord]

thanks @serbiant

[Morxander]

@serbiant I'm using the latest version 0.8.0 which includes your fix but it's still exposing null.

[norydev]

I think you broke something that was working.

Before this change:

class PostEntity < Grape::Entity
  expose :title
end

class CommentEntity < Grape::Entity
  expose :post, expose_nil: false do |object, options|
    PostEntity.represent(object.post, options)
  end
end

Now let's imagine a comment with no post for some reason, in that scenario, PostEntity.represent(object.post, options) is not nil, it's an instance of PostEntity. But it evaluates to nil as object.post is nil

This used to work fine before this change, but now it doesn't as it thinks the block does not return nil. I think you just check nil evaluation in a different place.