add nullable type annotations

Author: hden

aws-xray-recorder-sdk-core/src/main/java/com/amazonaws/xray/AWSXRayRecorder.java

@@ -141,7 +141,7 @@ public AWSXRayRecorder() {
         this(null);
     }
 
-    AWSXRayRecorder(Emitter sharedEmitter) {
+    AWSXRayRecorder(@Nullable Emitter sharedEmitter) {
         samplingStrategy = new DefaultSamplingStrategy();
         streamingStrategy = new DefaultStreamingStrategy();
         prioritizationStrategy = new DefaultPrioritizationStrategy();