When mongos is shutting down we prohibit creation of new transactions since we are draining the node from running commands.

This is implemented by a uassert that triggers before we initialize the transaction state.

However, usages of the transaction router may at times attempt to abort a transaction in case any error occurs, leading us to call TransactionRouter::implicitlyAbortTransaction in cases where we don't have the transaction initialized.

This results in a confusing yet harmless invariant failure since there is no transaction to abort.