raise assertion error if output is falsy

No error was being raised when output was {}, but the following line
"output.split('\n')" raised an AssertionError.

Change-Id: I37d27f8b890941d9333e71c64b1af6127505d766
diff --git a/tempest/api/compute/servers/test_server_actions.py b/tempest/api/compute/servers/test_server_actions.py
index 25df6e6..73cd13f 100644
--- a/tempest/api/compute/servers/test_server_actions.py
+++ b/tempest/api/compute/servers/test_server_actions.py
@@ -225,7 +225,7 @@
             resp, output = self.servers_client.get_console_output(
                 self.server_id, 10)
             self.assertEqual(200, resp.status)
-            self.assertIsNotNone(output)
+            self.assertTrue(output, "Console output was empty.")
             lines = len(output.split('\n'))
             self.assertEqual(lines, 10)
         self.wait_for(get_output)