Fix: Add muted Next and Previous links as fallback

[Rishit30G]

What?

Closes #69994

Why?

This PR is necessary to add muted 'Next' and 'Previous' links instead of not rendering any content on the frontend when Post Navigation Link Block is added to the first or the last post.

How?

This PR introduces a default $content which has Next and Previous Text along with the arrow

Testing Instructions

• Add a new post
• Add the Next Post Block to it
• Save the post and check the frontend
• Notice now that the Next text is rendered in the muted state instead of not appearing at all

Screenshots or screencast

Screen.Recording.2025-04-28.at.11.55.18.AM.mov

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Co-authored-by: Rishit30G <rishit30g@git.wordpress.org>
Co-authored-by: Infinite-Null <ankitkumarshah@git.wordpress.org>

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[Infinite-Null]

Hi @Rishit30G, I tested this PR and it works well for me 👍 :

I noticed that there are a few PHP coding standards issues that need to be addressed.

[Rishit30G]

Thanks @Infinite-Null for testing the changes!
I have resolved the PHPCS issues