Fix editor root triple click crash

[BitPhinix]

Description
Fixes a regression introduced by #4933. Basically, when triple-clicking the editor root, the path has a length of 0 so it was calling Editor.start,Editor.end with a path of [undefined].

This pr also moved some in-place checks for the later handling before the manual triple-click handling to ensure we are acting on a valid state.

Issue
Fixes: #4941

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: 58cb881

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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