bug(KeySync.$$handleUpdate): should not call normalize when value is undefined

[gjungb]

I experienced the following issue:

• create an object such as

page = { boxes: [{ paragraphs: [{ content: 'some text' }]}] }

• call

model.$add(page).then(function(result) { console.log(result.value); });

• result.value contains - as expected - the newly created page
• nonetheless KeySync.$$handleUpdate is called with a value of undefined
• the undefined value is passed to normalize(value, context)
• normalize seems to traverse the whole model setting all values to undefined => browser freezes and crashes