Validate the param idx, since zero is not a valid number

[pacu]

    // TODO: validate the idx, since zero is not a valid number
    // see: https://github.com/pacu/zcash-swift-payment-uri/issues/5
    static func parameterIndex(_ idx: UInt?) -> String {
        switch idx {
        case .some(let i):
            ".\(i)"
        case .none:
            ""
        }
    }