A potential NPE in spring module

[haotianw465]

https://github.com/aws/aws-xray-sdk-java/blob/master/aws-xray-recorder-sdk-spring/src/main/java/com/amazonaws/xray/spring/aop/AbstractXRayInterceptor.java#L50

If the subsegment is null when there is no tracing context, this line will cause an NPE. In short-term the subsegment capture needs to do null-check. In long-term the recorder should deprecate the Subsegment beginSubsegment() and force all callers to use Optional<Subsegment> beginSubsegment().

[haotianw465]

This issue is fixed as part of 2.1.0 release. See CHANGELOG for details.