Check if property is optional in union property template

Author: fyhertz

openapi_python_client/templates/property_templates/union_property.pyi

@@ -18,7 +18,14 @@ def _parse_{{ property.python_name }}(data: Dict[str, Any]) -> {{ property.get_t
     {% endif %}
     {% endfor %}
 
+{% if not property.nullable %}
 {{ property.python_name }} = _parse_{{ property.python_name }}({{ source }})
+{% else %}
+{{ property.python_name }} = None
+if {{ source }} is not None:
+    {{ property.python_name }} = _parse_{{ property.python_name }}({{ source }})
+{% endif %}
+
 {% endmacro %}
 
 {% macro transform(property, source, destination) %}