fixed ihm mmcif parsing

src/mol-model-formats/structure/mmcif/parser.ts

@@ -282,7 +282,7 @@ async function readIHM(ctx: RuntimeContext, format: mmCIF_Format, formatData: Fo
     const { ihm_model_list } = format.data;
     const entities: Entities = { data: format.data.entity, getEntityIndex: Column.createIndexer(format.data.entity.id) };
 
-    if (!format.data.atom_site.ihm_model_id.isDefined) {
+    if (format.data.atom_site._rowCount && !format.data.atom_site.ihm_model_id.isDefined) {
         throw new Error('expected _atom_site.ihm_model_id to be defined')
     }