Zoe: reallocating empty amounts for seats that had no prior allocation for the brand

[katelynsills]

The following test fails:

test(`zcfSeat.stage, zcf.reallocate with only empty amounts`, async t => {
  const { zcf } = await setupZCFTest();
  const { zcfSeat: zcfSeat1 } = zcf.makeEmptySeatKit();
  const { zcfSeat: zcfSeat2 } = zcf.makeEmptySeatKit();
  const zcfMint = await zcf.makeZCFMint('RUN');
  const { brand } = zcfMint.getIssuerRecord();

  // Get the amount allocated on zcfSeat1. It is empty for the RUN brand.
  const allocation = zcfSeat1.getAmountAllocated('RUN', brand);
  t.true(amountMath.isEmpty(allocation));

  // Stage zcfSeat2 with the allocation from zcfSeat1
  const zcfSeat2Staging = zcfSeat2.stage({ RUN: allocation });

  // Stage zcfSeat1 with empty
  const zcfSeat1Staging = zcfSeat1.stage({ RUN: amountMath.makeEmpty(brand) });

  zcf.reallocate(zcfSeat1Staging, zcfSeat2Staging);

  t.deepEqual(zcfSeat1.getCurrentAllocation(), {
    RUN: amountMath.make(0n, brand),
  });
  t.deepEqual(zcfSeat2.getCurrentAllocation(), {
    RUN: amountMath.make(0n, brand),
  });
});

It fails because reallocate checks that the number of brands before a reallocation and after a proposed reallocation are the same. This is meant to be a quick defense against a reallocation that allocates more tokens than existed previously, but it forgets about the possibility of empty amounts. We should allow for reallocations of empty amounts, even if the seats had no previous allocation for the brand.