[RFR] Login Background Image: handle empty ref

[Kmaschta]

Even though the React doc ensure the ref creation is done before the
componentDidMount, it can happen that the ref is set to null until the
next render.

So, to handle this case the component will now try to load the image on
the componentDidMount, but if the ref doesn't exist, it will try again
on the following componentDidUpdate. The try will be done only once.

Refs:

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

[djhi]

As discussed, let's avoid adding props on the Admin component. We should better document how to disable it using a custom Login page component

[Kmaschta]

Good point 👍
I added a doc in the theming too, but the section of the login page seems pretty empty.

If you want, I can add more points to the Using a Custom Login Page with more detailed examples (from Stack Overflow for example)