Fix agent NPE in annotation value mapper

[jaehong-kim]

Fix agent NPE in annotation value mapper

Error logs

java.lang.NullPointerException: null
	at com.navercorp.pinpoint.grpc.trace.PAnnotation$Builder.setValue(PAnnotation.java:509) ~[pinpoint-grpc-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.context.grpc.mapper.SpanMessageMapperImpl.map(SpanMessageMapperImpl.java:231) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.context.grpc.mapper.SpanMessageMapperImpl.map(SpanMessageMapperImpl.java:175) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.context.grpc.mapper.SpanMessageMapperImpl.map(SpanMessageMapperImpl.java:77) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.context.grpc.GrpcSpanMessageConverter.buildPSpan(GrpcSpanMessageConverter.java:81) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.context.grpc.GrpcSpanMessageConverter.toMessage(GrpcSpanMessageConverter.java:71) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.context.grpc.GrpcSpanMessageConverter.toMessage(GrpcSpanMessageConverter.java:40) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.sender.grpc.SpanGrpcDataSender$1.onDispatch(SpanGrpcDataSender.java:69) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.sender.grpc.SpanGrpcDataSender$1.onDispatch(SpanGrpcDataSender.java:66) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.sender.grpc.stream.DefaultStreamTask$2.dispatch(DefaultStreamTask.java:92) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at com.navercorp.pinpoint.profiler.sender.grpc.stream.DefaultStreamTask$2.run(DefaultStreamTask.java:78) ~[pinpoint-profiler-3.0.2.jar:3.0.2]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:572) ~[?:?]
	at java.util.concurrent.FutureTask.run(FutureTask.java:317) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[?:?]
	at java.lang.Thread.run(Thread.java:1583) [?:?]