Buttons: Fix the initial white space in nofollow rel

[vipul0425]

What?

Fixes: #66300

Why?

When the button settings don't specify a rel attribute value and the user clicks on 'Mark as nofollow' in the link's advanced settings, an extra space is added to the rel attribute in the button link.

How?

This PR trims the final value of rel attribute to remove redundant initial space.

Testing Instructions

1. Add a button block and insert a link
2. Edit the link and check the mark as nofollow option
3. View the resulting HTML code via the Code editor
4. OR inspect the button element via the browser
5. Notice the link's rel="nofollow" attribute should not have space before the value

Screenshots or screencast

Screen.Recording.2024-10-22.at.12.04.29.PM.mov

[github-actions]

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Co-authored-by: vipul0425 <vipulgupta003@git.wordpress.org>
Co-authored-by: akasunil <sunil25393@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: franzaurus <franz00@git.wordpress.org>

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[akasunil]

LGTM 🎉

[Mamaduka]

The getUpdatedLinkAttributes method has unit tests. Let's add coverage for this case as well.

[Mamaduka]

Thank you, @vipul0425 and @akasunil!