Yep, that whole example after the float cast is bit silly. After the cast, the value can never be an empty string, or null. Not to mention, comparing a floating point value to an arbitrary, literal value (of zero in this case) is potentially problematic [1].
[1]: See the warning on this page, this applies to all languages with floating point arithmetic. https://www.php.net/manual/en/language.types.float.php