[BUG] EuiButtonEmpty's iconSize prop is not escaped from the DOM which throws an error in the console

[ashwin-pc]

Describe the bug

Using the EuiButtonEmpty component's iconSize prop causes react to throw an error because the prop is passed down to the HTML

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior

A clear and concise description of what you expected to happen.

Screenshots

Host/Environment (please complete the following information):

• OUI Version: [e.g. 1.0]
• OSD Version (if applicable): [e.g. 2.7.0]
• OS: [e.g. iOS]
• Browser and version [e.g. Chrome 90]

Additional context

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