fix: handle NPE when NotifyCenter.INSTANCE is null during graceful shutdown

[Senrian]

What

When Spring context closes, NacosGracefulShutdownDelegate.doGracefulShutdown() calls autoServiceRegistration.stop() which internally deregisters subscribers from Nacos NotifyCenter. However, if the Nacos client shutdown hooks have already destroyed NotifyCenter.INSTANCE, this results in:

java.lang.NullPointerException: Cannot read field "sharePublisher" because "NotifyCenter.INSTANCE" is null

Fix

Add explicit NullPointerException catch in doGracefulShutdown() and log a warning instead of propagating, since this NPE is safe to ignore during shutdown when the NotifyCenter has already been cleaned up.

Testing

The existing test sameContextShouldTriggerStop and other tests in NacosGracefulShutdownDelegateTests continue to pass since they mock autoServiceRegistration.stop() to not throw NPE.

Fixes #4283

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