lang: funcs: Send empty values when appropriate

I seem to have forgotten to differentiate between the empty string and
no data because the zero value for the stored result was the empty
string. This turns it into a pointer so that we don't block the function
engine if a template or one of the other patched functions sends an
empty string as the first value.

lang/funcs/simplepoly/simplepoly.go

@@ -280,6 +280,7 @@ func (obj *WrappedFunc) Stream() error {
 				obj.init.Logf("Function returned with: %+v", values)
 			}
 
+			// TODO: do we want obj.result to be a pointer instead?
 			if obj.result == result {
 				continue // result didn't change
 			}