Initialize an empty executed_operations on MapTask

This is needed as it may be accessed by the reporting
thread before the operations are fully populated.

----Release Notes----
[]
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=116896643

Author: robertwb

google/cloud/dataflow/worker/batchworker.py

@@ -491,7 +491,6 @@ def run(self):
             with work_item.lock:
               self.set_current_work_item_and_executor(
                   work_item, executor.MapTaskExecutor())
-              work_item.map_task.executed_operations = []
               self.report_completion_status(
                   work_item, exception_details=deferred_exception_details)
               work_item.done = True

google/cloud/dataflow/worker/maptask.py

@@ -551,6 +551,7 @@ def __init__(self, operations, stage_name, step_names):
     self.operations = operations
     self.stage_name = stage_name
     self.step_names = step_names
+    self.executed_operations = []
 
   def __str__(self):
     return '<%s %s steps=%s>' % (self.__class__.__name__, self.stage_name,