set_default: Allow empty vectors

This is an assumption that's existed since the Summary Page was first
devised---that all vectors have at least one value.  This is because the
bucket (originating from Liza) always has at least one value in its vectors.

Of course, we still have a problem in that the Summary Page initializes
everything to have a single value by default, and that's still the
case.  But this will at least allow for things _outside_ the Summary Page to
provide an empty array.  I'll have to address the Summary Page separately,
and that's going to be difficult, since we don't really want to change the
behavior across the board.

* src/current/compiler/js.xsl (set_defaults): Default max index to 0 if
    `length' is unavailable, rather than 1.

src/current/compiler/js.xsl

@@ -1932,7 +1932,7 @@
         input = input || [];
 
         // vector or matrix
-        var i = input.length || 1;
+        var i = input.length || 0;
         var ret = [];
         var value = ( depth === 2 ) ? [ value ] : value;