VMware: fix compute node exception when no hosts in cluster

Commit 4033c0c9c1 casues the
regression. The result was a empty string and not None.
That is suds returns a Text object and not None.

Change-Id: I79e70e300f40eb3561cf56478d578f0f8cda273e
Closes-bug: #1383305
(cherry picked from commit 419096ec19)

nova/tests/virt/vmwareapi/test_ds_util.py

@@ -311,6 +311,16 @@ class DsUtilTestCase(test.NoDBTestCase):
                               ds_util.get_datastore,
                               self.session, 'fake-cluster')
 
+    def test_get_datastore_no_host_in_cluster(self):
+        def fake_call_method(module, method, *args, **kwargs):
+            return ''
+
+        with mock.patch.object(self.session, '_call_method',
+                               fake_call_method):
+            self.assertRaises(exception.DatastoreNotFound,
+                              ds_util.get_datastore,
+                              self.session, 'fake-cluster')
+
     def _test_is_datastore_valid(self, accessible=True,
                                  maintenance_mode="normal",
                                  type="VMFS",

nova/virt/vmwareapi/ds_util.py

@@ -248,7 +248,9 @@ def get_datastore(session, cluster, datastore_regex=None):
                                 vim_util,
                                 "get_dynamic_property", cluster,
                                 "ClusterComputeResource", "datastore")
-    if datastore_ret is None:
+    # If there are no hosts in the cluster then an empty string is
+    # returned
+    if not datastore_ret:
         raise exception.DatastoreNotFound()
 
     data_store_mors = datastore_ret.ManagedObjectReference