splitting.monotonic: return array from empty trends.

basis/splitting/monotonic/monotonic-tests.factor

@@ -30,6 +30,8 @@ USING: tools.test math arrays kernel sequences ;
 { { { 3 3 } } }
 [ { 3 3 } [ = ] slice monotonic-slice [ >array ] map ] unit-test
 
+{ { } } [ "" trends ] unit-test
+
 {
     {
         T{ upward-slice { from 0 } { to 3 } { seq { 1 2 3 2 1 } } }

basis/splitting/monotonic/monotonic.factor

@@ -62,5 +62,5 @@ TUPLE: upward-slice < slice ;
         [ stable-slices ]
         [ upward-slices ] tri 3append [ from>> ] sort-with
     ] [
-        zero? [ ] [ [ 0 1 ] dip stable-slice boa ] if
+        zero? [ drop { } ] [ [ 0 1 ] dip stable-slice boa ] if
     ] if ;