Only apply Firefox toSlatePoint() offset fix when the cloned contents end in \n\n

[clauderic]

Description
This PR follows up on #4547.

Context
I meant to update the PR to only apply the offset fix when the cloned textContent ends in \n\n instead of \n but didn't get a chance to push that up before it was merged.

The rationale for this change is that we only need to apply this fix for the specific edge-case where Firefox returns \n\n for the textContent of the cloned range.

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: 1469fbf

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

This PR includes changesets to release 1 package

Name	Type
slate	Patch

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