tweak EXTREMUM

tweak EXTREMUM

 Return NIL if the sequence is empty, instead of the NO-EXTREMUM nonsense.

 It was bad design, because it's not an error someone higher up the stack can
 sensibly handle, and handling it locally is too verbose and slow.

    (or (extremum ...) (error ...))

 expresses the common case succintly, and fits the pattern of existing
 sequence functions.

 If it is deemed necessary, we can also add &KEY DEFAULT, but that seems
 overkill and has little precedent in sequence functions.