iAPI: Don't error when data-wp-each gets undefined

[luisherranz]

Description

If data-wp-each gets a undefined it just throws.

Instead of that, it should subscribe to the property and treat it as an empty array until it not longer undefined.

Step-by-step reproduction instructions

Just use data-wp-each with a non-existing value:

<template data-wp-each="context.nonExistingValue">