HHH-15270 Fix inconsistent precedence of orm.xml implicit catalog over "default_catalog" in XML-mapped entities

hibernate-core/src/main/java/org/hibernate/boot/model/source/internal/hbm/ModelBinder.java

@@ -2971,7 +2971,7 @@ public class ModelBinder {
 			return database.toIdentifier( tableSpecSource.getExplicitCatalogName() );
 		}
 		else {
-			return database.toIdentifier( metadataBuildingContext.getMappingDefaults().getImplicitCatalogName() );
+			return null;
 		}
 	}