* This is passed to the ZAP handler in the 'domain' field
* If not set, or empty, then NULL security does not call the ZAP handler
* This resolves the phantom ZAP request syndrome seen with sockets where
security was never intended (e.g. in test cases)
* This means if you install a ZAP handler, it will not get any requests
for new connections until you take some explicit action, which can be
setting a username/password for PLAIN, a key for CURVE, or the domain
for NULL.
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