squash! fixup! improv: Check reference prop types for all customs

Fix property dictionary emptiness check for custom extensions. Neither
`_properties` or `_top_level_properties` (if provided) may be empty.

stix2/registration.py

@@ -163,16 +163,22 @@ def _register_extension(
                 ext_type,
             )
 
-    # Need to check both toplevel and nested properties
-    combined_props = {
-        **new_extension._properties,
-        **getattr(new_extension, "_toplevel_properties", dict()),
-    }
-    if not combined_props:
+    tl_props = getattr(new_extension, "_toplevel_properties", None)
+    if any((
+        # There must always be at least one property in an extension. This
+        # holds for instances of both custom object extensions which must
+        # contain one or more custom properties, and `extension-definition`s
+        # which must contain an `extension_type` property.
+        not new_extension._properties,
+        # If a top-level properties mapping is provided, it cannot be empty
+        tl_props is not None and not tl_props,
+    )):
         raise ValueError(
             "Invalid extension: must define at least one property: " +
             ext_type,
         )
+    # We need to validate all properties related to this extension
+    combined_props = dict(new_extension._properties, **(tl_props or dict()))
     _validate_props(combined_props, version)
 
     EXT_MAP = registry.STIX2_OBJ_MAPS[version]['extensions']