Add empty dataframe functionality

[hannahblair]

Description

This PR

• Ensures dataframes can be empty
• Adds an add row button when there are zero rows
• Ensures cell menu buttons can be used in headers (regression)
• Fixes some issues with the table border and the border radius

Closes: #10568

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[gradio-pr-bot]

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Install Gradio from this PR

pip install https://gradio-pypi-previews.s3.amazonaws.com/b20af0eebb7b5ec46668aef89eb17f4e15e89216/gradio-5.16.0-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@b20af0eebb7b5ec46668aef89eb17f4e15e89216#subdirectory=client/python"

Install Gradio JS Client from this PR

npm install https://gradio-npm-previews.s3.amazonaws.com/b20af0eebb7b5ec46668aef89eb17f4e15e89216/gradio-client-1.12.0.tgz

Use Lite from this PR

<script type="module" src="https://gradio-lite-previews.s3.amazonaws.com/b20af0eebb7b5ec46668aef89eb17f4e15e89216/dist/lite.js""></script>

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@gradio/dataframe	patch
gradio	patch

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With the following changelog entry.

Add empty dataframe functionality

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[pngwn]

This works great from my testing and code looks great too! Thanks @hannahblair!