api_documentation: Display null data type if potential value.

If null is a potential value of data type for a return value or
parameter in the API endpoint, then it is rendered as an option.

This currently relies on the 'nullable' setting in the OpenAPI spec
that was removed in the 3.1.0 release. If/when the OpenAPI version
is updated, then how the `data_type` for parameters and return values
is rendered will need to be reworked.

Fixes #20264.

zerver/lib/markdown/api_arguments_table_generator.py

@@ -191,4 +191,6 @@ def generate_data_type(schema: Mapping[str, Any]) -> str:
         data_type = "(" + generate_data_type(schema["items"]) + ")[]"
     else:
         data_type = schema["type"]
+        if "nullable" in schema and schema["nullable"]:
+            data_type = data_type + " | null"
     return data_type