Do not raise error when body is empty and nil content type

Empty response bodies (204 responses for instance) should not have a
content type and therefore an error should not be raised in strict mode
when the response body is empty and the content type is nil as this
behavior is expected.

lib/openapi_parser/schemas/response.rb

@@ -21,7 +21,9 @@ def validate(response_body, response_validate_options)
 
       media_type = select_media_type(response_body.content_type)
       unless media_type
-        raise ::OpenAPIParser::NotExistContentTypeDefinition, object_reference if response_validate_options.strict
+        if response_validate_options.strict && response_body_not_blank(response_body)
+          raise ::OpenAPIParser::NotExistContentTypeDefinition, object_reference
+        end
 
         return nil
       end
@@ -39,6 +41,11 @@ def select_media_type(content_type)
 
     private
 
+      # @param [OpenAPIParser::RequestOperation::ValidatableResponseBody]
+      def response_body_not_blank(response_body)
+        !(response_body.response_data.nil? || response_body.response_data.empty?)
+      end
+
       # @param [Hash] response_headers
       def validate_header(response_headers)
         return unless headers

spec/openapi_parser/request_operation_spec.rb

@@ -67,7 +67,7 @@
       let(:content_type) { nil }
       let(:data) { { 'string' => 1 } }
 
-      it { expect(subject).to eq nil }
+      it { expect(subject).to eq true }
     end
 
     context 'with header' do
@@ -159,6 +159,7 @@
 
           context 'not exist content type' do
             let(:content_type) { 'application/xml' }
+            let(:data) { '<something></something>' }
 
             it do
               expect { subject }.to raise_error do |e|
@@ -167,6 +168,18 @@
               end
             end
           end
+
+          context 'with nil content type when the response body is blank' do
+            let(:status_code) { 204 }
+            let(:content_type) { nil }
+            let(:data) { '' }
+            let(:http_method) { :get }
+
+            it do
+              expect { subject }.to_not raise_error
+              expect(subject).to eq true
+            end
+          end
         end
 
         context 'default parameter' do
@@ -193,6 +206,7 @@
 
           context 'not exist content type' do
             let(:content_type) { 'application/xml' }
+            let(:data) { '<something></something>' }
 
             it do
               expect { subject }.to raise_error do |e|