Do not attempt to set `.paren` on `undefined`.

- Set `.paren` on the comma expression itself, not the last sub-
  expression.
- Having `.exprs` be enumerable makes for easier debugging, so just do
  the simple thing and set the property directly.
- Clean up the code a little to use a `ret` temp var.
Guard against commas containing `undefined` in `checkCondAssignment`.

- Adjust checking to match new `.paren` policy in `prefix("(", ...)`.
Extend test for empty conditions to check various invalid conditions.