Fix problem in previous param types branch with return output params.

In ```{? = CALL fun(?) }```, the zeroth param is special and should not trigger an assertion.
This is equivalent to ```VALUES(fun($2))```, with no use of ```$1```, which was covered properly. The difference is that the first includes the return param in the list.
So, an alternative to the approach taken here would be to remove the param from the list when the ```AST``` is constructed, for instance.