### Body

Currently `start_date` is mandatory parameter.
For DAGs with `schedule=None` we can relax this requirement as no scheduling calculation needed so the `start_date` parameter isn't used.

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
"""Test that @dag uses function docs as doc_md for DAG object if doc_md is not explicitly set."""
        @dag_decorator(default_args=self.DEFAULT_ARGS, doc_md=dag_doc_md)
        def noop_pipeline():
            """Regular DAG documentation"""
        dag = noop_pipeline()
        assert isinstance(dag, DAG)
        assert dag.dag_id == "noop_pipeline"
        assert dag.doc_md == expected_doc_md
    def test_documentation_template_rendered(self):
        """Test that @dag uses function docs as doc_md for DAG object"""
        @dag_decorator(default_args=self.DEFAULT_ARGS)
        def noop_pipeline():
            """
            {% if True %}
               Regular DAG documentation
            {% endif %}
            """
        dag = noop_pipeline()
        assert isinstance(dag, DAG)
        assert dag.dag_id == "noop_pipeline"
        assert "Regular DAG documentation" in dag.doc_md
    def test_resolve_documentation_template_file_rendered(self, tmp_path):