Created attachment 464579 [details]
Demonstrates the error thrown with a string passed and none without.

I believe by spec toLocaleLowerCase / toLocaleUpperCase are required to throw a RangeError if you specify an invalid locale per section 9.2.1 CanicalizeLocaleList with the exception thrown in step 7.c.v (https://tc39.es/ecma402/#sec-canonicalizelocalelist)

Firefox throws a RangeError for both of the following:
''.toLocaleLowerCase('');
''.toLocaleUpperCase('');

Chrome throws a RangeError for ''.toLocaleUpperCase('') and before M109 did for ''.toLocaleLowerCase(''), the lack of error for lowercase is tracked as https://crbug.com/1409058

Safari does not throw an error if the string being converted is empty, but does if you have a non-empty string being converted.