Editor: Apply IE11 input fix only when mounting TinyMCE

[aduth]

Related: #12234 (comment)

This pull request seeks to resolve an error which will occur when unmounting an instance of the RichText component in IE11. As of #12234, the ref callback handler was not accounting for the null value being passed during unmounting of the component, thus resulting in errors when subsequent logic would attempt to reference properties as if it were an DOM node.

React will call the ref callback with the DOM element when the component mounts, and call it with null when it unmounts.

https://reactjs.org/docs/refs-and-the-dom.html#callback-refs

Testing instructions:

In Internet Explorer 11 and your preferred browser, ensure that no errors are logged when removing a paragraph block:

1. Navigate to Posts > Add New
2. Click the writing prompt
3. Press Backspace