Fix null value in ResultPath (#48)

* Fix null value in ResultPath

Fixes #45

* Add unit test covering default ResultPath

* Fix null values in InputPath and OutputPath

Author: tuliocasagrande

Diff for: src/stepfunctions/steps/states.py

@@ -66,9 +66,10 @@ def _replace_placeholders(self, params):
 
     def to_dict(self):
         result = {}
+        fields_accepted_as_none = ('result_path', 'input_path', 'output_path')
         # Common fields
         for k, v in self.fields.items():
-            if v is not None:
+            if v is not None or k in fields_accepted_as_none:
                 k = to_pascalcase(k)
                 if k == to_pascalcase(Field.Parameters.value):
                     result[k] = self._replace_placeholders(v)

Diff for: tests/unit/test_steps.py

@@ -346,4 +346,43 @@ def test_retry_fail_for_unsupported_state():
     c1 = Choice('My Choice')
 
     with pytest.raises(ValueError):
-        c1.add_catch(Catch(error_equals=["States.NoChoiceMatched"], next_step=Fail("ChoiceFailed")))
+        c1.add_catch(Catch(error_equals=["States.NoChoiceMatched"], next_step=Fail("ChoiceFailed")))
+
+
+def test_paths_none():
+    task_state = Task('Task', resource='arn:aws:lambda:us-east-1:1234567890:function:StartLambda',
+                      result_path=None,
+                      input_path=None,
+                      output_path=None)
+    assert 'ResultPath' in task_state.to_dict()
+    assert task_state.to_dict()['ResultPath'] is None
+
+    assert 'InputPath' in task_state.to_dict()
+    assert task_state.to_dict()['InputPath'] is None
+
+    assert 'OutputPath' in task_state.to_dict()
+    assert task_state.to_dict()['OutputPath'] is None
+
+
+def test_paths_none_converted_to_null():
+    task_state = Task('Task', resource='arn:aws:lambda:us-east-1:1234567890:function:StartLambda',
+                      result_path=None,
+                      input_path=None,
+                      output_path=None)
+    assert '"ResultPath": null' in task_state.to_json()
+    assert '"InputPath": null' in task_state.to_json()
+    assert '"OutputPath": null' in task_state.to_json()
+
+
+def test_default_paths_not_included():
+    task_state = Task('Task', resource='arn:aws:lambda:us-east-1:1234567890:function:StartLambda')
+    assert 'ResultPath' not in task_state.to_dict()
+    assert 'InputPath' not in task_state.to_dict()
+    assert 'OutputPath' not in task_state.to_dict()
+
+
+def test_default_paths_not_converted_to_null():
+    task_state = Task('Task', resource='arn:aws:lambda:us-east-1:1234567890:function:StartLambda')
+    assert '"ResultPath": null' not in task_state.to_json()
+    assert '"InputPath": null' not in task_state.to_json()
+    assert '"OutputPath": null' not in task_state.to_json()