Fix text content not rendered on ssr

[hueyhe]

Description
The fix makes sure that the text content of the TextString component is rendered on the first render.

Issue
Fixes: server-side rendering issue introduced by #4733.
Correct fix of #4796.

Example
Please see example in #4796.

Context
#4733 spellcheck fix changed the render behavior of the TextString component. TextString won't render the actual text content on the first-time render, which means server-side rendering is broken. This fix ensures that the text content will get rendered immediately on the first-time render.

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: 0e7515a

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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