perf: speedup of CBLSLazyPublicKey::operator== when comparing to the default / null object; speedup CDeterministicMNList::AddMN by avoiding check to IsValid when a nullcheck is sufficient

perf: speedup of CBLSLazyPublicKey::operator== when comparing to the default / null object; speedup CDeterministicMNList::AddMN by avoiding check to IsValid when a nullcheck is sufficient #1836