Let error(null) throw null

This patch removes the weird behaviour of jv_invalid_with_msg(jv_null())
that returns jv_invalid() (i.e. empty), instead of a boxed jv_null().

The previous behaviour of  null|error  was obviously unintentional, and
allowing is jv_invalid_with_msg() to return values on which you can't
call jv_invalid_get_msg() is only error prone.

docs/content/manual/manual.yml

@@ -1231,10 +1231,6 @@ sections:
           given as the argument. Errors can be caught with try/catch;
           see below.
 
-          When the error value is `null`, it produces nothing and works
-          just like `empty`. So `[null | error]` and `[error(null)]` both
-          emit `[]`.
-
         examples:
           - program: 'try error catch .'
             input: '"error message"'