Fixes CLI client called without subcommands

If CLI client is called without any subcommands or arguments it will
fail with """'Namespace' object has no attribute 'command'""". This
is coming from the getattr which does not have alternate value
specified.

Closes-Bug: #1494259

Change-Id: I461f0d4a91f3af2224bafc14a88572a8e4a3c051

glanceclient/tests/unit/test_shell.py

@@ -158,6 +158,13 @@ class ShellTest(testutils.TestCase):
             self.assertEqual(0, actual)
             self.assertFalse(et_mock.called)
 
+    def test_blank_call(self):
+        shell = openstack_shell.OpenStackImagesShell()
+        with mock.patch.object(shell, '_get_endpoint_and_token') as et_mock:
+            actual = shell.main('')
+            self.assertEqual(0, actual)
+            self.assertFalse(et_mock.called)
+
     def test_help_on_subcommand_error(self):
         self.assertRaises(exc.CommandError, shell,
                           '--os-image-api-version 2 help bad')