Don't crash when an operation returns a null value

[cowboyd]

We need to check for resources on return values of operations in order to link them to the parent context. To do this, we check that the object can receive property access. However, typeof null is actually "object".

This adjusts the check to cover both undefined and null (as well as any other object type)

[github-actions]

A preview package of this pull request has been published with the tag null-operations.
You can try it out by running the following command:

$ npm install effection@null-operations

or by updating your package.json to:

{
  "effection": "null-operations"
}

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