Table "Merge" button shows even if user selected a single cell

[KorovinQuantori]

Description

Steps to Reproduce

1. Go to https://platejs.org/docs/table (Table merge section)
2. Add some node (for example an image to an empty cell
3. Merge a cell with image and some other cell with text
4. Manually select content in the merged cell (both text and image should be in the selection, screenshot below)
   
5. Merge button appears

Sandbox

Expected Behavior

Expecting merge button to appear only if user selected more than one cell

Environment

• slate:
• slate-react:
• browser: chrome

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