fix: toSlatePoint suppressThrow leaf without text node

[BitPhinix]

Description
toSlatePoint throws while using suppressThrow if the selection is inside a node with a data-slate-leaf attribute with no text child (due to calling range.setStart with textNode being null). This can happen when e.g. rendering a slate 0.47 editor inside a void element.

This pr adds a simple check to see if textNode is set.

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: 0ef59a1

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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[BitPhinix]

Ci passes locally