fix #2070: error while listing mobile nav apps

Author: ludomikula

server/api-service/lowcoder-domain/src/main/java/org/lowcoder/domain/application/model/Application.java

@@ -242,7 +242,7 @@ public Mono<Object> getLiveContainerSize(ApplicationRecordService applicationRec
             if (ApplicationType.APPLICATION.getValue() == getApplicationType()) {
                 return Mono.empty();
             }
-            return Mono.just(getContainerSizeFromDSL(dsl));
+            return Mono.justOrEmpty(getContainerSizeFromDSL(dsl));
         });
     }