Stubbing interfaces/objects seems to not support overriding members with null value

See - https://github.com/forcedotcom/ts-sinon/blob/6dcb6055fd9064cd42159ab38e94be7a8f2dcab6/src/index.ts#L34

Currently, the workflow of stubbing an interface with optional members does not really seem to work as expected. For example, if I have an interface that is similar to this:

```
interface Test {
testMember: MemberObject;
}
```

Where `MemberObject` is an arbitrary object, calling the following does not override the value of the stubbed object with null as one may expect:

`tsSinon.stubInterface(sinon, { testMember: null })`

For my testing purposes, I'd like to verify that specific members of an object exist and are non-null, however currently this override is ignored, and proceeds to create a stub for this, which seems like incorrect behavior:
https://github.com/forcedotcom/ts-sinon/blob/6dcb6055fd9064cd42159ab38e94be7a8f2dcab6/src/index.ts#L43

It falls through to this because the call to `makeProxyGet` hardcodes stubMissing === true from the call via `stubInterface`. As such, overriding a member with a null value is not possible, and will incorrectly return a stub.

Is this extension library capable of supporting this, or is this in fact an issue with the current implementation?

Thank you!