Fix span dispatch failed

[jaehong-kim]

Fix span dispatch fail

Related log.

dispatch failed

java.lang.NullPointerException: null
	at com.navercorp.pinpoint.grpc.trace.PAcceptEvent$Builder.setRpc(PAcceptEvent.java:665) ~[pinpoint-grpc-2.3.3.jar:2.3.3]
	at com.navercorp.pinpoint.profiler.context.grpc.GrpcSpanMessageConverter.newAcceptEvent(GrpcSpanMessageConverter.java:175) ~[pinpoint-profiler-2.3.3.jar:2.3.3]
	at com.navercorp.pinpoint.profiler.context.grpc.GrpcSpanMessageConverter.buildPSpan(GrpcSpanMessageConverter.java:115) ~[pinpoint-profiler-2.3.3.jar:2.3.3]
	at com.navercorp.pinpoint.profiler.context.grpc.GrpcSpanMessageConverter.toMessage(GrpcSpanMessageConverter.java:90) ~[pinpoint-profiler-2.3.3.jar:2.3.3]
	at com.navercorp.pinpoint.profiler.context.grpc.GrpcSpanMessageConverter.toMessage(GrpcSpanMessageConverter.java:60) ~[pinpoint-profiler-2.3.3.jar:2.3.3]
	at com.navercorp.pinpoint.profiler.sender.grpc.SpanGrpcDataSender$1.onDispatch(SpanGrpcDataSender.java:59) ~[pinpoint-profiler-2.3.3.jar:2.3.3]
	at com.navercorp.pinpoint.profiler.sender.grpc.SpanGrpcDataSender$1.onDispatch(SpanGrpcDataSender.java:56) ~[pinpoint-profiler-2.3.3.jar:2.3.3]
	at com.navercorp.pinpoint.profiler.sender.grpc.stream.DefaultStreamTask$2.dispatch(DefaultStreamTask.java:92) [pinpoint-profiler-2.3.3.jar:2.3.3]
	at com.navercorp.pinpoint.profiler.sender.grpc.stream.DefaultStreamTask$2.run(DefaultStreamTask.java:78) [pinpoint-profiler-2.3.3.jar:2.3.3]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_202]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_202]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_202]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_202]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_202]