Hotfix: unnecessary Discard Changes dialog for RTEs

[madsrasmussen]

Fixes: #17670

The PR aims to clean up how we set/update the RTE property value so we don't get the "Discard Changes" dialog unless there has been user updates to the value.

How I went about this:

Allow the internal property _value to be undefined.
The property value can be undefined but the internal _value could only be the full value object with blocks. By aligning these it is easier to keep them in sync. The new code clears all internal the values and only sets the block data if we get a markup value.

Handles the Tip Tap empty <p></p> tag (ueberdosis/tiptap#154)
This issue wasn't directly linked to this issue but for us to be able to check for an empty tip tap editor I encountered this issue. Tip Tap has a property on their element to check if it's empty. It ignores the

and returns the value you would expect. I have added a public method on our element to proxy their value.

What to test:

• Please ensure that you only get the "Discard Changes"-dialog if you have done any changes to an RTE.
• Test both Tiny MCE and Tip Tap

[iOvergaard]

Tested it out and looks like it is working.

[NguyenThuyLan]

Hi, I realize it also happens on Content pages that have BlockList/BlockGrid inside, is this PR fixed that issue also? @madsrasmussen

[engern]

I got the same issue @NguyenThuyLan and created a new issue for it: #17784