Revert "set_default: Allow empty vectors"

I need to revert this for now because it breaks YAML test cases.  The proper
fix is a more expressive type system with dependent types that would allow
it to know the proper number of indexes to initialize relative to other
inputs.  I wanted to implement this anyway to help catch iteration-related
bugs.

I'm tabling this for now, though, since I have other things that I need to
work on.

This reverts commit 4406cbe553.

src/current/compiler/js.xsl

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