fix(Transaction): discard empty transactions on CommitWith

[Wondertan]

Problems

• Transactions with empty pendingWrites were never discarded(and marked as done) for CommitWith
  • This is similar to fix(Transactions): Fix resource consumption on empty write transaction #2018, which solved the same problem for Commit, but not for CommitWith
  • The CommitWith is used by WriteBatch, so flushing an empty batch never discarded the inner shadowed transaction, causing a multitude of issues.

Solution

Make sure Discard is called for CommitWith when pendingWrites are empty. The existing unit test is updated to assert that.

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[Wondertan]

cc @billprovince @joshua-goldstein

[Wondertan]

cc @harshil-goel @mangalaman93

[Wondertan]

Ok, so I think I misunderstood the purpose of precommitCheck, and transactions should not be discarded when it errors. I still think the code in Commit funcs is rather confusing and could be simplified to have a single defer, but that's definitely not part of this PR, so I shortened the solution to a one-liner, as well updated unit tests that checks for the bug

[Wondertan]

Ping again