Allow default exposed value to be false or any empty data (#371)

Using `present?` limits what can be set as `:default` and restricts
falsy values like `false`, `''`, `[]` for example.

I don't really see why not allow those (I have a use case for `false`
for example, which is to ensure a nullable boolean value is always
rendered as a boolean).

Co-authored-by: peter scholz <pscholz.le@gmail.com>

Author: norydev

CHANGELOG.md

@@ -7,6 +7,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#371](https://github.com/ruby-grape/grape-entity/pull/371): Allow default exposed value to be `false` or any empty data - [@norydev](https://github.com/norydev).
 
 * [#352](https://github.com/ruby-grape/grape-entity/pull/369): Remove `FetchableObject` behavior. - [@danielvdao](https://github.com/danielvdao).
 

lib/grape_entity/exposure/base.rb

@@ -89,7 +89,7 @@ def valid_value(entity, options)
           return unless valid?(entity)
 
           output = value(entity, options)
-          output.blank? && @default_value.present? ? @default_value : output
+          output.blank? && !@default_value.nil? ? @default_value : output
         end
 
         def should_return_key?(options)

spec/grape_entity/entity_spec.rb

@@ -247,9 +247,9 @@ def initialize(a, b, c)
           context 'when default option is set' do
             it 'exposes default values for attributes' do
               subject.expose(:a, default: 'a')
-              subject.expose(:b, default: 'b')
+              subject.expose(:b, default: false)
               subject.expose(:c, default: 'c')
-              expect(subject.represent(model).serializable_hash).to eq(a: 'a', b: 'b', c: 'value')
+              expect(subject.represent(model).serializable_hash).to eq(a: 'a', b: false, c: 'value')
             end
           end