Fix generic param for generate_default_from_enum_variant

[A4-Tacks]

Input:

//- minicore: default
enum Variant<T> {
    Defined(T),
    Undefi$0ned,
}

Old:

enum Variant<T> {
    Defined(T),
    Undefined,
}

impl Default for Variant {
    fn default() -> Self {
        Self::Undefined
    }
}

This PR fixed:

enum Variant<T> {
    Defined(T),
    Undefined,
}

impl<T> Default for Variant<T> {
    fn default() -> Self {
        Self::Undefined
    }
}