[Go] Fix generated client tests when there is no response body (#14081)

* [WIP] Isolated test case for Go api_test generator

* Fix tests for API endpoints without a return type

* Add the rest of the generated test fix

Author: ctreatma

modules/openapi-generator/src/main/resources/go/api_test.mustache

@@ -39,10 +39,12 @@ func Test_{{packageName}}_{{classname}}Service(t *testing.T) {
 		var {{paramName}} {{{dataType}}}
 		{{/pathParams}}
 
-		resp, httpRes, err := apiClient.{{classname}}.{{operationId}}(context.Background(){{#pathParams}}, {{paramName}}{{/pathParams}}).Execute()
+		{{#returnType}}resp, {{/returnType}}httpRes, err := apiClient.{{classname}}.{{operationId}}(context.Background(){{#pathParams}}, {{paramName}}{{/pathParams}}).Execute()
 
 		require.Nil(t, err)
+		{{#returnType}}
 		require.NotNil(t, resp)
+		{{/returnType}}
 		assert.Equal(t, 200, httpRes.StatusCode)
 
 	})

modules/openapi-generator/src/test/java/org/openapitools/codegen/go/GoClientCodegenTest.java

@@ -261,4 +261,29 @@ public void verifyFormatErrorMessageInUse() throws IOException {
         TestUtils.assertFileContains(Paths.get(output + "/api_pet.go"),
                 "newErr.error = formatErrorMessage(localVarHTTPResponse.Status, &v)");
     }
+
+    @Test
+    public void verifyApiTestWithNullResponse() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("go")
+                .setGitUserId("OpenAPITools")
+                .setGitRepoId("openapi-generator")
+                .setInputSpec("src/test/resources/3_0/go/petstore-with-no-response-body.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        TestUtils.assertFileExists(Paths.get(output + "/test/api_pet_test.go"));
+        TestUtils.assertFileNotContains(Paths.get(output + "/test/api_pet_test.go"),
+                "require.NotNil(t, resp)");
+        TestUtils.assertFileNotContains(Paths.get(output + "/test/api_pet_test.go"),
+                "resp, httpRes, err := apiClient.PetApi.PetDelete(context.Background()).Execute()");
+        TestUtils.assertFileContains(Paths.get(output + "/test/api_pet_test.go"),
+                "httpRes, err := apiClient.PetApi.PetDelete(context.Background()).Execute()");
+    }
 }

modules/openapi-generator/src/test/resources/3_0/go/petstore-with-no-response-body.yaml

@@ -0,0 +1,22 @@
+openapi: 3.0.0
+info:
+  description: >-
+    This spec is mainly for testing Petstore server and contains fake endpoints,
+    models. Please do not use this for any other purpose. Special characters: "
+    \
+  version: 1.0.0
+  title: OpenAPI Petstore
+  license:
+    name: Apache-2.0
+    url: 'https://www.apache.org/licenses/LICENSE-2.0.html'
+tags:
+  - name: pet
+    description: Everything about your Pets
+paths:
+  /pet:
+    delete:
+      tags:
+        - pet
+      responses:
+        '204':
+          description: OK