Add missing @nullable to fix the build (#15474)

Author: laurit

instrumentation/spring/spring-kafka-2.7/library/src/main/java/io/opentelemetry/instrumentation/spring/kafka/v2_7/InstrumentedRecordInterceptor.java

@@ -138,6 +138,6 @@ public void clearThreadState(Consumer<?, ?> consumer) {
 
   private static class ThreadState {
     // used to record the error in failure() so it could be used in afterRecord()
-    Throwable error;
+    @Nullable Throwable error;
   }
 }