Fix locker.py

Author: ashnair1

src/poetry/packages/locker.py

@@ -612,7 +612,7 @@ def _dump_package(self, package: Package) -> dict:
             if package.source_resolved_reference:
                 data["source"]["resolved_reference"] = package.source_resolved_reference
 
-            if package.source_subdirectory:
+            if hasattr(package, "source_subdirectory"):
                 data["source"]["source_subdirectory"] = package.source_subdirectory
 
             if package.source_type in ["directory", "git"]: