code: def test_annotation_post_schema_start_dttm_null() -> None:
      with pytest.raises(ValidationError):
          result = AnnotationPostSchema().load(
              {
                  "short_descr": "foo",
                  "start_dttm": None,
                  "end_dttm": END_STR,
              }
          )
  
imports: import pytest
  from marshmallow.exceptions import ValidationError
  from superset.annotation_layers.annotations.schemas import AnnotationPostSchema, AnnotationPutSchema
  from superset.annotation_layers.schemas import AnnotationLayerPostSchema, AnnotationLayerPutSchema
  from tests.unit_tests.annotation_layers.fixtures import END_DTTM, END_STR, START_DTTM, START_STR
label_desc: Use image node_id 11 for calling a global function with example usage: test_annotation_post_schema_start_dttm_null() without return types