[FEATURE] UI: centralise `ComponentRenderer::render()`

[thibsy]

Hi folks,

This is one part of the PR I created to introduce JavaScript hydration to the UI framework (#6948).

It centralises the ComponentRenderer::render() function inside our AbstractComponentRenderer by using a new abstract method called renderComponent() and gets rid of the AbstractComponentRenderer::checkComponent() calls. I also tried to streamline the instanceof checks a bit, and returned null in methods which threw a LogicException or returned an empty string if the component could not be handled.

Kind regards, @thibsy