Skip empty entity tests for grape-swagger < 2.1.3

grape-swagger < 2.1.3 doesn't support empty model definitions.
Support was added in grape-swagger PR #963 which will be released
in version 2.1.3. Tests are skipped for older versions.

Author: numbata

spec/issues/962_polymorphic_entity_with_custom_documentation_spec.rb

@@ -1,12 +1,17 @@
 # frozen_string_literal: true
 
 describe '#962 empty entity with custom documentation type' do
+  # grape-swagger < 2.1.3 doesn't support empty model definitions (PR #963)
+  let(:supports_empty_models?) { Gem::Version.new(GrapeSwagger::VERSION) >= Gem::Version.new('2.1.3') }
+
   context 'when entity has no properties' do
     subject(:swagger_doc) do
       get '/swagger_doc'
       JSON.parse(last_response.body)
     end
 
+    before { skip 'grape-swagger < 2.1.0 does not support empty models' unless supports_empty_models? }
+
     let(:app) do
       Class.new(Grape::API) do
         namespace :issue962 do
@@ -65,6 +70,8 @@ class Report < Grape::Entity
       JSON.parse(last_response.body)
     end
 
+    before { skip 'grape-swagger < 2.1.0 does not support empty models' unless supports_empty_models? }
+
     let(:app) do
       Class.new(Grape::API) do
         namespace :issue962 do