Remove type condition (#1558)

In cases where the empty object is already set as default in the config:

See: `stubs`, `mocks` and `methods`
https://github.com/vuejs/vue-test-utils/blob/dev/packages/test-utils/src/config.js

The ? is not necessary. In this case, we can import the config and we will not be forced to check the existence of attributes:

```
import { config } from '@vue/test-utils';

if (config.mocks) {
  config.mocks.i18n = { t: jest.fn() };
  config.mocks.$t = jest.fn();
}
```

and use directly

```
import { config } from '@vue/test-utils';

config.mocks.i18n = { t: jest.fn() };
config.mocks.$t = jest.fn();
```

packages/test-utils/types/index.d.ts

@@ -160,9 +160,9 @@ type ShallowMountOptions<V extends Vue> = MountOptions<V>
 type ThisTypedShallowMountOptions<V extends Vue> = ShallowMountOptions<V> & ThisType<V>
 
 interface VueTestUtilsConfigOptions {
-  stubs?: Record<string, Component | boolean | string>
-  mocks?: Record<string, any>
-  methods?: Record<string, Function>
+  stubs: Record<string, Component | boolean | string>
+  mocks: Record<string, any>
+  methods: Record<string, Function>
   provide?: Record<string, any>,
   silent?: Boolean,
   showDeprecationWarnings?: boolean