add nullaway

Author: zeitlinger

instrumentation-api-incubator/src/main/java/io/opentelemetry/instrumentation/api/incubator/semconv/db/DbClientCommonAttributesGetter.java

@@ -16,11 +16,7 @@ default String getSystem(REQUEST request) {
     return null;
   }
 
-  // TODO: make this required to implement
-  @Nullable
-  default String getDbSystem(REQUEST request) {
-    return getSystem(request);
-  }
+  String getDbSystem(REQUEST request);
 
   @Deprecated
   @Nullable