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

### 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:

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