Update properties.md

[danny-andrews]

Correct note on converting from attribute to property.

Attributes can't be null. They can only be strings.

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[justinfagnani]

Thanks for the PR!

Attributes can be null in that getAttribute() returns null if the value isn't present: https://developer.mozilla.org/en-US/docs/Web/API/Element/getAttribute#non-existing_attributes. The phrasing could just say if the attribute is present or not, though I'd rather check and change the language in the new lit.dev site.