no need to write an empty audit log entry (#13854)

Author: andrewstalin

ydb/core/audit/audit_log_impl.cpp

@@ -179,7 +179,9 @@ class TAuditLogActor final : public TActor<TAuditLogActor> {
                 ? AuditLogItemBuilders[builderIndex] : AuditLogItemBuilders[DefaultAuditLogItemBuilder];
             const auto msg = ev->Get();
             const auto auditLogItem = builder(msg->Time, msg->Parts);
-            WriteLog(auditLogItem, logBackends.second);
+            if (!auditLogItem.empty()) {
+                WriteLog(auditLogItem, logBackends.second);
+            }
         }
     }