Fix AssertionError when ServiceTalk context captured with no activeSpan (#8951)

Author: ygree

dd-java-agent/instrumentation/servicetalk/servicetalk-0.42.56/src/main/java/datadog/trace/instrumentation/servicetalk0_42_56/DatadogCapturedContextProvider.java

@@ -16,7 +16,11 @@ public CapturedContext captureContext(CapturedContext underlying) {
 
   @Override
   public CapturedContext captureContextCopy(CapturedContext underlying) {
-    return new WithDatadogCapturedContext(AgentTracer.activeSpan(), underlying);
+    AgentSpan activeSpan = AgentTracer.activeSpan();
+    if (activeSpan == null) {
+      return underlying;
+    }
+    return new WithDatadogCapturedContext(activeSpan, underlying);
   }
 
   private static final class WithDatadogCapturedContext implements CapturedContext {

dd-java-agent/instrumentation/servicetalk/servicetalk-0.42.56/src/test/groovy/ContextPreservingInstrumentationTest.groovy

@@ -27,6 +27,27 @@ class ContextPreservingInstrumentationTest extends AgentTestRunner {
     assertParentChildTrace()
   }
 
+  def "capturedContext without an active span"() {
+    when:
+    runInSeparateThread {
+      try (def _ = asyncContextProvider.captureContext().attachContext()) {
+        childSpan()
+      }
+    }
+
+    then:
+    assertTraces(1) {
+      trace(1) {
+        span {
+          operationName "child"
+          tags {
+            defaultTags()
+          }
+        }
+      }
+    }
+  }
+
   def "wrapBiConsumer"() {
     setup:
     def parent = startParentContext()