Test start_workers/stop_workers with trivial args

Add a test to ensure that `start_workers` can be passed `n = 0` without
issues. Also ensure this translates into no workers being added. Further
test that `stop_workers` can be called with an empty dictionary.

dask_drmaa/tests/test_core.py

@@ -15,6 +15,16 @@
 from distributed.utils import tmpfile
 
 
+def test_no_workers(loop):
+    with DRMAACluster(scheduler_port=0) as cluster:
+        with Client(cluster, loop=loop) as client:
+            cluster.start_workers(0)
+            assert not cluster.workers
+            cluster.stop_workers({})
+
+    assert not os.path.exists(cluster.script)
+
+
 def test_simple(loop):
     with DRMAACluster(scheduler_port=0) as cluster:
         with Client(cluster, loop=loop) as client: