return non for owner and group fsobject methods on Windows

Author: tclose

fileformats/core/mixin.py

@@ -626,10 +626,11 @@ def register_converter(
                     f"from a generic type or similarly classified type, not {source_format}"
                 )
             else:
-                if cls.wildcard_classifiers() != source_format.wildcard_classifiers():  # type: ignore[attr-defined]
+                source_wildcard_classifiers = source_format.wildcard_classifiers()  # type: ignore[attr-defined]
+                if cls.wildcard_classifiers() != source_wildcard_classifiers:
                     raise FormatDefinitionError(
                         f"Mismatching wildcards between source format, {source_format} "
-                        f"({list(source_format.wildcard_classifiers())}), and target "  # type: ignore[attr-defined]
+                        f"({list(source_wildcard_classifiers)}), and target "
                         f"{cls} ({cls.wildcard_classifiers()})"
                     )
                 prev_registered = [

fileformats/generic/fsobject.py

@@ -58,8 +58,11 @@ def drive(self) -> str:
     def exists(self) -> bool:
         return True
 
-    def group(self) -> str:
-        return self.fspath.group()
+    def group(self) -> ty.Optional[str]:
+        try:
+            return self.fspath.group()
+        except AttributeError:
+            return None
 
     def is_dir(self) -> bool:
         return self.fspath.is_dir()
@@ -71,8 +74,11 @@ def is_file(self) -> bool:
     def name(self) -> str:
         return self.fspath.name
 
-    def owner(self) -> str:
-        return self.fspath.owner()
+    def owner(self) -> ty.Optional[str]:
+        try:
+            return self.fspath.owner()
+        except AttributeError:
+            return None
 
     @property
     def parent(self) -> Path: