Protect intercepted null traces from NPE (#9797)

* Protect intercepted null traces from NPE

* Remove bad check

Author: amarziali

dd-trace-core/src/main/java/datadog/trace/core/CoreTracer.java

@@ -13,6 +13,7 @@
 import static datadog.trace.common.metrics.MetricsAggregatorFactory.createMetricsAggregator;
 import static datadog.trace.util.AgentThreadFactory.AGENT_THREAD_GROUP;
 import static datadog.trace.util.CollectionUtils.tryMakeImmutableMap;
+import static java.util.Collections.emptyList;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.concurrent.TimeUnit.MINUTES;
 import static java.util.concurrent.TimeUnit.NANOSECONDS;
@@ -1183,7 +1184,11 @@ private List<DDSpan> interceptCompleteTrace(List<DDSpan> trace) {
           String interceptorName = interceptor.getClass().getName();
           rlLog.warn("Throwable raised in TraceInterceptor {}", interceptorName, e);
         }
+        if (interceptedTrace == null) {
+          interceptedTrace = emptyList();
+        }
       }
+
       trace = new ArrayList<>(interceptedTrace.size());
       for (final MutableSpan span : interceptedTrace) {
         if (span instanceof DDSpan) {

dd-trace-core/src/test/groovy/datadog/trace/core/TraceInterceptorTest.groovy

@@ -171,6 +171,29 @@ class TraceInterceptorTest extends DDCoreSpecification {
     tags.size() >= 7
   }
 
+  def "should be robust when interceptor return a null trace"() {
+    setup:
+    tracer.interceptors.add(new TraceInterceptor() {
+        @Override
+        Collection<? extends MutableSpan> onTraceComplete(Collection<? extends MutableSpan> trace) {
+          null
+        }
+
+        @Override
+        int priority() {
+          return 0
+        }
+      })
+
+    when:
+    DDSpan span = (DDSpan) tracer.startSpan("test", "test")
+    span.phasedFinish()
+    tracer.write([span])
+
+    then:
+    notThrown(Throwable)
+  }
+
   def "register interceptor through bridge"() {
     setup:
     GlobalTracer.registerIfAbsent(tracer)