[11.x] Handle HtmlString constructed with a null

[sperelson]

I managed to construct an HtmlString with a null value. I realise that inserting a null value into an HtmlString is pretty bad and I should have sanitized the value properly beforehand.

However, since this is possible, I believe that the __toString() and isEmpty methods should gracefully handle null values. This will prevent a type error exception when getting the string value and correctly reflect that the HtmlString is empty.

[rodrigopedra]

I should have sanitized the value properly beforehand.

Yes. HtmlString's constructor type hint (in its docblock) says it only deals with strings.

Even if parameters type hints were used, you'd have to sanitize any values before passing down to the constructor.