Refactor: Empty string handling as extension

[Root-Core]

Split refactoring part from #216

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• The current implementation is more error prone and verbose.
• The refactor is a bit more polished, safe and has less boilerplate.
• The refactor is explicit in casting the type string to null if empty.

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• While I appreciate the refactor, I personally think StringUtil.EmptyToNull(...) is a little more clear on what it does versus the .OrNull() extension.

One last try. :)
I renamed it to ToNullIfEmpty(), as it casts to null. This is more clear and the To part indicates the type change.

• And I'm generally a fan of extension methods :) The repo is full of them.

You are right, I missed that.. I moved the extension into the existing StringExtensions.cs.

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Disclaimer: No AI was used in this pull request.