test(): unexpected stringification of variables assigned to process.env

After validation, environment variables are assigned back to
process.env. This leads to stringification of the values. This
stringification is deprecated and might result in a runtime error in
the future. See https://nodejs.org/api/process.html#processenv.

A special case is when an environment variable has the value
`undefined`. This causes `ConfigService.get` to fall back on
process.env thereby returning the stringified form `'undefined'`.

tests/e2e/optional.spec.ts

@@ -0,0 +1,33 @@
+import { Test } from '@nestjs/testing';
+import { AppModule } from '../src/app.module';
+import { ConfigService } from '../../lib';
+
+describe('Optional environment variables', () => {
+  it('should return undefined for optional variables', async () => {
+    const module = await Test.createTestingModule({
+      imports: [AppModule.withValidateFunction(() => ({
+        optional: undefined,
+      }))],
+    }).compile();
+
+    const app = module.createNestApplication();
+    await app.init();
+
+    const optional = module.get(ConfigService).get('optional')
+
+    expect(optional).toEqual(undefined)
+  });
+
+  it('should not assign complex objects back to process.env', async () => {
+    const module = await Test.createTestingModule({
+      imports: [AppModule.withValidateFunction(() => ({
+        complex: {hello: 'there'},
+      }))],
+    }).compile();
+
+    const app = module.createNestApplication();
+    await app.init();
+
+    expect(process.env.complex).toEqual(undefined)
+  });
+});