Change task() function to return 'null' if task doesn't exist

* Before this change function task(task_name) available in Mistral
  expressions would fail with an error if such task doesn't exist.
  It makes impossible to have expressions like <% task(t1) != null %>
  which is very convenient in cases when we need to gather meta
  info about how a workflow was running and what tasks have already
  started and what have not. So now it simply returns 'null'.

Change-Id: Ifd1ea9b5ee946c7072044d1402296710676845db
Closes-Bug: #1631035

mistral/tests/unit/engine/test_yaql_functions.py

@@ -108,6 +108,46 @@ class YAQLFunctionsEngineTest(engine_test_base.EngineTestCase):
             task2.published
         )
 
+    def test_task_function_returns_null(self):
+        wf_text = """---
+            version: '2.0'
+
+            wf:
+              output:
+                task2: <% task(task2) %>
+                task2bool: <% task(task2) = null %>
+
+              tasks:
+                task1:
+                  action: std.noop
+                  on-success:
+                    - task2: <% false %>
+
+                task2:
+                  action: std.noop
+        """
+
+        wf_service.create_workflows(wf_text)
+
+        wf_ex = self.engine.start_workflow('wf', {})
+
+        self.await_workflow_success(wf_ex.id)
+
+        with db_api.transaction():
+            wf_ex = db_api.get_workflow_execution(wf_ex.id)
+
+            self.assertDictEqual(
+                {
+                    'task2': None,
+                    'task2bool': True
+                },
+                wf_ex.output
+            )
+
+            task_execs = wf_ex.task_executions
+
+        self.assertEqual(1, len(task_execs))
+
     def test_task_function_non_existing(self):
         wf_text = """---
             version: '2.0'

mistral/utils/expression_utils.py

@@ -138,9 +138,7 @@ def task_(context, task_name):
         task_ex = task_execs[-1] if len(task_execs) > 0 else None
 
     if not task_ex:
-        raise ValueError(
-            'Failed to find task execution with name: %s' % task_name
-        )
+        return None
 
     # We don't use to_dict() db model method because not all fields
     # make sense for user.