Make _updateCollectionData null-safe

[sionzee]

When you scope your data eg.
let $scope = model.scope('_page.variable') And set it to null
$scope.set(null);
In method react-sharedb/lib/subscribe.js: _updateCollectionData, will be called:
this.data = (0, _observerUtil.observable)(this.data);
Where this.data == null;
In observer methods it will test if it is not instrument and call on it:
var constructor = ref.constructor Where ref will be null.

Temporary fix for this is to set false insteadof null.

[cray0000]

good catch. Though it's better to use model.del() for such operations. I'll take a look at how to patch it.