feat: support optional domain fields (nil verifying_contract)

[mdon]

Problem

Passing nil for verifying_contract in the Domain struct crashes with FunctionClauseError in EIP712.Util.pad/2:

%EIP712.Typed{
  domain: %EIP712.Typed.Domain{
    chain_id: 137,
    name: "ClobAuthDomain",
    version: "1"
    # verifying_contract defaults to nil
  },
  ...
}
|> EIP712.Typed.encode()
# ** (FunctionClauseError) no function clause matching in EIP712.Util.pad/2

Per the EIP-712 spec, domain fields like verifyingContract and salt are optional — protocols should be able to omit them from the domain type hash entirely.

This is needed for protocols like Polymarket whose ClobAuth domain only has 3 fields (name, version, chainId) without verifyingContract.

Changes

• domain_type/1 now accepts a Domain struct and only includes non-nil fields in the type definition (used for the type hash)
• serialize_keys/1 filters out nil-valued fields instead of including them
• domain_seperator/1 passes the domain struct to domain_type/1
• domain_type/0 (no args) still returns all fields for backwards compatibility

Verification

• All 75 existing doctests pass unchanged
• Tested with Polymarket's ClobAuth domain (3-field, no verifyingContract) — domain separator matches the Python py-clob-client reference implementation