Issue #3619901: Document the negotiated domain as nullable (4.x)

Carries the #3619901 correction to this branch.

DomainNegotiationContext::$domain starts as NULL and is only ever set by setDomain(), so everything handing it back can return NULL: on the CLI, during cron, and whenever no domain matches the request. The class says so itself three lines under the getter, where hasDomain() is $this->domain !== NULL.

DomainNegotiatorInterface::getActiveDomain() was already correct on this branch. These three were not:

• DomainNegotiationContext::$domain, the property the chain returns.
• DomainNegotiationContext::getDomain(), which returns it directly.
• DomainSourcePathProcessor::getActiveDomain(), which reads the same context.

No behavior change and no signature change. The same correction, plus the interface, landed on 3.0.x as 71ce3b1d.