Fix NPE in PayloadDispatcherImpl

Author: amarziali

dd-trace-core/src/main/java/datadog/trace/common/writer/PayloadDispatcherImpl.java

@@ -68,7 +68,7 @@ public void addTrace(List<? extends CoreSpan<?>> trace) {
     // there are alternative approaches to avoid blocking here, such as
     // introducing an unbound queue and another thread to do the IO
     // however, we can't block the application threads from here.
-    if (null == mapper || !packer.format(trace, mapper)) {
+    if (null == mapper || null == packer || !packer.format(trace, mapper)) {
       healthMetrics.onFailedPublish(
           trace.isEmpty() ? 0 : trace.get(0).samplingPriority(), trace.size());
     }