SpringBoot TomcatMetricsBinder may log a warning for an NPE if the context is closed before the ApplicationStartedEvent is published

fix for NPE in my unit test.

2020-06-28 10:40:25.832 - org.springframework.beans.factory.support.DisposableBeanAdapter -467378 [SpringContextShutdownHook] WARN  [] - Invocation of destroy method failed on bean with name 'tomcatMetricsBinder': java.lang.NullPointerException

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