Popover: don't render fallback anchor if anchorRef is defined

[ellatrix]

Description

Currently a span element is always rendered for a Popover to use as a fallback if there is no anchorRef or anchorRect. It's not needed to render this element if either of these is defined.

How has this been tested?

Popover should render as before. I also tested that the scroll lock still works for the inserter on small screens.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR. .

[ellatrix]

I also tested with Storybook that the fallback still works.