Fix accidental hard dependency on dask in BackfillJob (#8025)

PR #7786 pulled this back in in such a way that made dask a hard
dependency on `airflow backfill` commands.

airflow/jobs/backfill_job.py

@@ -30,7 +30,6 @@
     AirflowException, DagConcurrencyLimitReached, NoAvailablePoolSlot, PoolNotFound,
     TaskConcurrencyLimitReached,
 )
-from airflow.executors.dask_executor import DaskExecutor
 from airflow.executors.local_executor import LocalExecutor
 from airflow.executors.sequential_executor import SequentialExecutor
 from airflow.jobs.base_job import BaseJob
@@ -773,6 +772,12 @@ def _execute(self, session=None):
 
         # picklin'
         pickle_id = None
+
+        try:
+            from airflow.executors.dask_executor import DaskExecutor
+        except ImportError:
+            DaskExecutor = None
+
         if not self.donot_pickle and \
                 self.executor.__class__ not in (LocalExecutor, SequentialExecutor, DaskExecutor):
             pickle = DagPickle(self.dag)