add a seat in zcf.mintGains if one isn't provided

[Chris-Hibbert]

What is the Problem Being Solved?

zcf.mintGains() allows the seat parameter to be defaulted, but doesn't currently fill it in when missing.

   // TODO unimplemented
   assert(
     zcfSeat !== undefined,
     details`On demand seat creation not yet implemented`,
   );

Description of the Design

use a mechanism like makeEmptySeatKit() (or maybe use makeEmptySeatKit()) to fill in a seat when it's not present

Security Considerations

This doesn't add extra vulnerabilities to Zoe, but it's an extra mechanism in contracts that makes it possible to create a seat and then lose track of it, losing assets.

Test Plan

add a test case in test-zcf.js