Update Audio constructor test to match specs

Neither HTML nor WebIDL seem to say anything that would make Audio()
without new work. Even the very first revision of Web Apps 1.0 from
2006 says "The Audio() *constructor* takes a single argument" and
nothing about calling it as a function. Ad-hoc testing found that
presently Gecko, Presto, and Trident support calling Audio() without
new, while Blink and WebKit do not.

Also test null and undefined as the arguments, since an undefined
optional argument should be treated as a missing argument per WebIDL,
but isn't in all implementations.