Handle enum variants without type clauses

[ericnordelo]

Issue:

Currently, enum variants without type clauses panic when processed with:

Error: Unexpected Type node kind: TypeClause

For example:

enum ProposalState {
    Pending,
    Active,
    Canceled,
    Defeated,
    Succeeded,
    Queued,
    Executed
}

Workaround solution:

We can set the empty type to these variants to compute the interface ID:

enum ProposalState {
    Pending: (),
    Active: (),
    Canceled: (),
    Defeated: (),
    Succeeded: (),
    Queued: (),
    Executed: (),
}