createStubInstance override and null return values

[spicalous]

Describe the bug
sinon.createStubInstance was extended recently to allow overrides such as:

sinon.createStubInstance(AClass, {
  aMethod: "a string",
  aChainableMethod: sinon.stub().returnsThis()
});

Theres a small issue which prevents a user from returning null values

To Reproduce

sinon.createStubInstance(AClass, {
  aMethod: null
});

Expected behavior
The library should correctly create a stub instance with a stub method returning null

Reason
Theres an initial check whether the override is a stub by checking if createStubInstance property exists on the return value https://github.com/sinonjs/sinon/blob/master/lib/sinon/stub.js#L85

This throws when we try to check a property of null

Context (please complete the following information):

• Library version: Current latest v7.x
• Environment: OSX

Thanks!
Let me know if it makes sense, I can help create a PR for it

[mroderick]

Thank you for your detailed analysis of the bug, it looks like an interesting one!
A pull request would be very welcome 👍