bpo-40334: Correctly generate C parser when assigned var is None (GH-20296)

When there are 2 negative lookaheads in the same rule, let's say `!"(" blabla "," !")"`, there will the 2 `FunctionCall`'s where assigned value is None. Currently when the `add_var` is called
the first one will be ignored but when the second lookahead's var is sent to dedupe it
will be returned as `None_1` and this won't be ignored by the declaration generator in the `visit_Alt`. This patch adds an explicit check to `add_var` to distinguish whether if there is a variable or not.