render void spacer in readonly mode

[BitPhinix]

Description
Currently finding the dom range of a void node in read-only crashes the editor since slate doesn't render the void spacer in read-only.

This pr simply adjusts the Element component so that it is rendered even in read-only.

⚠️ I'm not sure why this wasn't the case before, so there might be something I'm missing here.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: 21ab7c7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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[dylans]

I'm not sure why this wasn't the case before, so there might be something I'm missing here.

I'm guessing oversight rather than intentional. I'll land this now, thanks.