Prevent history checkpoints during backspace in empty editor

[Nantris]

Please describe your changes

Prevent running clearNodes when there's no nodes to clear to avoid useless history checkpoints

How did you accomplish your changes

Check if the first node in editor is a paragraph and if so, return false

How have you tested your changes

Locally

How can we verify your changes

Locally and checking editor.state.history$.done.eventCount

Remarks

It would be better if there were some schema.nodes.defaultNode, but in lieu of that I've assumed paragraph is the default.

Checklist

• The changes are not breaking the editor
• Added tests where possible
• Followed the guidelines
• Fixed linting issues

Related issues

#5062

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