fix[OA-audit-N09]: Missing check for non-zero address

[md0x]

Signed-off-by: Pablo Maldonado pablo@umaproject.org

Motivation

OZ identifies the following issue:

The _isDisputeAllowed function in the OptimisticAsserter contract may call
isDisputeAllowed on an optional EscalationManagerInterface contract. In
practice, the policy is to not validate disputers when no escalation manager is specified, so the
function will exit early instead of invoking a function on the zero address.
Nevertheless, in the interest of local reasoning, consider adding a zero-address check on the
em address prior to using it to make external function calls. Also consider swapping line 449
and line 450 so that the em address assignment is only made when this variable is required.

Summary

Add non-zero address check and return early

[mrice32]

LGTM!