Guard against null logback events (#11960)

Guard against null logback events

Co-authored-by: andrea.marziali <andrea.marziali@datadoghq.com>

Author: amarziali

dd-java-agent/instrumentation/logback-1.0/src/main/java/datadog/trace/instrumentation/logback/LogbackLoggerInstrumentation.java

@@ -74,6 +74,9 @@ public void methodAdvice(MethodTransformer transformer) {
   public static class CallAppendersAdvice {
     @Advice.OnMethodEnter(suppress = Throwable.class)
     public static void onEnter(@Advice.Argument(0) ILoggingEvent event) {
+      if (event == null) {
+        return;
+      }
       AgentSpan span = activeSpan();
 
       if (span != null && traceConfig(span).isLogsInjectionEnabled()) {
@@ -86,6 +89,9 @@ public static void onEnter(@Advice.Argument(0) ILoggingEvent event) {
   public static class CallAppendersAdvice2 {
     @Advice.OnMethodEnter(suppress = Throwable.class)
     public static void onEnter(@Advice.Argument(0) ILoggingEvent event) {
+      if (event == null) {
+        return;
+      }
       LogsIntakeHelper.log(event);
     }
   }