feat: Add a test case to require-valid-default-prop

[aryehb]

The documentation for Vue props, as well as the require-valid-default-prop rule, are not clear if setting a default value of null to a prop with type Object requires a factory function:

props: {
  foo: { type: Object, default: () => null },
},

vs.

props: {
  foo: { type: Object, default: null },
},

On one hand, typeof null === 'object'. On the other hand, the reason for using a factory function is so each instance of the prop has a different object reference, whereas null is always the same reference anyway.

This adds a valid test case for setting a default value of null to a prop with type Object without a factory function. The test passes, so it's evident that null does not require a factory function.

[FloEdelmann]

Looks good to me, thank you!