Fails when promiseGetter returns null

[green3g]

These lines fail https://github.com/canjs/can-stache-key/blob/master/can-stache-key.js#L190-L192

when a promise getter returns null, because typeof null is 'object' and null.then causes an error.

Example, I might not want to return a promise when there is no id to view, so I return null:

    focusObjectPromise: {
        get () {
            if (this.viewId) {
                const params = {};
                params[this.view.connection.idProp] = this.viewId;
                const promise = this.view.connection.get(params);

                promise.catch((err) => {
                    dev.warn('unable to complete focusObject request', err);
                });

                return promise;
            }
            return null;
        }
    },