Prevent RTE being marked as dirty when content contains empty paragraph

[mcl-sz]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #18127 #16922

Description

In Tinymce, an empty paragraph is filled with a non-breaking space character. When saving the content, the DOMParser replaces this empty non-breaking space character with an   entity. This causes the editor to be dirty when the content contains an empty paragraph. By making these the same before the comparison, this is prevented.

Tesing

• In Umbraco 13 add a Richtext editor (Umbraco.TinyMCE) to a Doctype.
• Optionally add an Blockgrid editor and add an element that contains an Richtext editor.
• Create a page of this doctype and put 2 lines of text in the editor and separate them with an empty paragraph (simply push Enter).
• Save the page.
• Click inside the editor and then click outside the editor.
• Navigate to another page, a "Unsaved changes"-dialog will appear, even without changing anything.

Apply the patch and try the last 2 steps again. The "Unsaved changes"-dialog doesn't appear anymore.

[github-actions]

Hi there @mcl-sz, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
• If this is a new feature, Umbraco HQ provided guidance on the implementation beforehand
• 💡 The contribution looks original and the contributor is presumably allowed to share it

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[mcl-sz]

It's not entirely clear to me whether I should use the "dev" or "main" branch to submit the change.

[AndyButland]

Thanks @mcl-sz, it does look like this solves the problem. I've followed along with your testing steps and tried a few more combinations, and am no longer seeing the incorrect dialog for pending changes.