test(config): add unit test for generic config service

tests/e2e/optional-generic.spec.ts

@@ -0,0 +1,40 @@
+import { INestApplication } from '@nestjs/common';
+import { Test, TestingModule } from '@nestjs/testing';
+import { AppModule } from '../src/app.module';
+import { ConfigService } from '../../lib';
+
+describe('Optional Generic()', () => {
+  let app: INestApplication;
+  let module: TestingModule;
+
+  beforeEach(async () => {
+    module = await Test.createTestingModule({
+      imports: [AppModule.withEnvVars()],
+    }).compile();
+
+    app = module.createNestApplication();
+    await app.init();
+  });
+
+  it(`should allow a key of the interface`, () => {
+    const configService = module.get<ConfigService<{ PORT: string }>>(
+      ConfigService,
+    );
+
+    const port = configService.get('PORT');
+
+    expect(port).toBeTruthy();
+  });
+
+  it(`should allow any key without a generic`, () => {
+    const configService = module.get<ConfigService>(ConfigService);
+
+    const port = configService.get('PORT');
+
+    expect(port).toBeTruthy();
+  });
+
+  afterEach(async () => {
+    await app.close();
+  });
+});