TASK: docs and readability of Node::getProperty

[mhsdesign]

This pr incorporates a few cleanups regarding node.properties

Clean up how node::getProperty and hasProperty access the lower level methods.

Make the code more obvious - calling node::getProperty and hasProperty goes into deep rabbit hole until we find the actual implementation. This makes it harder to understand the actual behavior. This also lead to slightly miss documented code:
For example hasProperty might be true for a property even if it is null, because a serializer might unserialize it to null!

I also adjusted the documentation (meta #4478)
As its was actually the same from the old cr and we dont have those funny content object dingsis anymore

Upgrade instructions

Review instructions

Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions

[mhsdesign]

might get obsolete when we opt in for @bwaidelich's cleanup #4304 (comment) / or can be build upon

[dlubitz]

You're right. I was assuming `isset` will return true if a key exists.

[bwaidelich]

@mhsdesign you have replaced the PropertyCollectionInterface with this PR, but it seems to be stuck on the mock-issue.
I created a PR now for just the interface removal with #4513 in order to slice up the task a little.
We should continue our discussion on how to best mock parts of the CR in our weekly tomorrow

[mhsdesign]

Rebased and removed all the crazy parts ;) so now its only improved docs and types and stuff ^^

[kdambekalns]

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