Fix NPE in rmi server instrumentation

[laurit]

When running the app from #4982 with javaagent noticed the following exception in debug log

[otel.javaagent 2022-01-07 13:05:14:257 +0200] [main] DEBUG io.opentelemetry.javaagent.bootstrap.ExceptionLogger - Failed to handle exception in instrumentation for javax.management.remote.rmi.RMIServerImpl on jdk.internal.loader.ClassLoaders$AppClassLoader@7ad041f3
java.lang.NullPointerException: Cannot invoke "io.opentelemetry.javaagent.shaded.io.opentelemetry.context.Context.get(io.opentelemetry.javaagent.shaded.io.opentelemetry.context.ContextKey)" because "context" is null
	at io.opentelemetry.javaagent.shaded.instrumentation.api.internal.SpanKey.fromContextOrNull(SpanKey.java:70)
	at io.opentelemetry.javaagent.shaded.instrumentation.api.instrumenter.SuppressIfSameSpanKeyStrategy.shouldSuppress(SuppressIfSameSpanKeyStrategy.java:33)
	at io.opentelemetry.javaagent.shaded.instrumentation.api.instrumenter.CompositeSuppressionStrategy.shouldSuppress(CompositeSuppressionStrategy.java:54)
	at io.opentelemetry.javaagent.shaded.instrumentation.api.instrumenter.Instrumenter.shouldStart(Instrumenter.java:138)
	at java.management.rmi/javax.management.remote.rmi.RMIServerImpl.setMBeanServer(RMIServerImpl.java:139)
	at java.management.rmi/javax.management.remote.rmi.RMIConnectorServer.start(RMIConnectorServer.java:450)
	at jdk.management.agent/sun.management.jmxremote.ConnectorBootstrap.startLocalConnectorServer(ConnectorBootstrap.java:583)
	at jdk.management.agent/jdk.internal.agent.Agent.startLocalManagementAgent(Agent.java:318)
	at jdk.management.agent/jdk.internal.agent.Agent.startAgent(Agent.java:450)
	at jdk.management.agent/jdk.internal.agent.Agent.startAgent(Agent.java:599)