Array join is defined as:

    declare function array:join($arrays as array(*)*) as array(*) {
              if (fn:count($arrays) lt 2)
              then $arrays
              else op:array-concat(fn:head($arrays), array:join(fn:tail($arrays)))
            };

if $arrays has count zero (i.e. $arrays is the empty sequence), then this definition says we return the empty sequence which is not permitted by the static return type, however the example

The expression array:join(()) returns [ ].

shows the intention is that the empty array is returned.