Ignore additionalItems when items isn't present on 2019.

This is specified behavior, see json-schema-org/JSON-Schema-Test-Suite#643.

CHANGELOG.rst

@@ -8,6 +8,7 @@ v4.18.0
   Please file issues on the ``referencing`` tracker if there is functionality missing from it, or here on the ``jsonschema`` issue tracker if you have issues with existing code not functioning the same, or with figuring out how to change it to use ``referencing``.
 * Support for Python 3.7 has been dropped, as it is nearing end-of-life.
   This should not be a "visible" change in the sense that ``requires-python`` has been updated, so users using 3.7 should still receive ``v4.17.3`` when installing the library.
+* On draft 2019-09, ``unevaluatedItems`` now properly does *not* consider items to be evaluated by an ``additionalItems`` schema if ``items`` is missing from the schema, as the specification says in this case that ``additionalItems`` must be completely ignored.
 
 v4.17.3
 =======

jsonschema/_legacy_validators.py

@@ -231,9 +231,6 @@ def find_evaluated_item_indexes_by_schema(validator, instance, schema):
         return []
     evaluated_indexes = []
 
-    if "additionalItems" in schema:
-        return list(range(0, len(instance)))
-
     if "$ref" in schema:
         resolved = validator._resolver.lookup(schema["$ref"])
         evaluated_indexes.extend(
@@ -248,6 +245,9 @@ def find_evaluated_item_indexes_by_schema(validator, instance, schema):
         )
 
     if "items" in schema:
+        if "additionalItems" in schema:
+            return list(range(0, len(instance)))
+
         if validator.is_type(schema["items"], "object"):
             return list(range(0, len(instance)))
         evaluated_indexes += list(range(0, len(schema["items"])))