fix uninitialized ASN.1 objects use

The underlying pyasn1 library has a notion of `schema` and
`value` object. The `schema` object can't be used with
regular Python operators as values because `schema` does
not carry any value, it's rather a type.

This fix turns a couple of `NoSuchInstance` schema
objects into proper `value` objects to retain compatibility
with the latest pysnmp which removed the default initializer
of the `NoSuchInstance` object.

Change-Id: Id044a0e437724bf1f3581e9187e2a1e6c336b1bc

virtualpdu/tests/integration/pdu/test_pdu.py

@@ -32,7 +32,7 @@ class TestPDU(PDUTestCase):
                           self.snmp_get, enterprises + (42,))
 
     def test_set_unknown_oid(self):
-        self.assertEqual(NoSuchInstance(),
+        self.assertEqual(NoSuchInstance(''),
                          self.snmp_set(enterprises + (42,), univ.Integer(7)))
 
     def test_get_valid_oid_wrong_community(self):

virtualpdu/tests/unit/test_pysnmp_handler.py

@@ -108,7 +108,7 @@ class SnmpServiceMessageReceivedTest(unittest.TestCase):
         varbindlist = message[PDUs][ResponsePDU][VarBindList]
         self.assertEqual(varbindlist[0][ObjectName].value, (1, 1))
         self.assertEqual(varbindlist[0][NoSuchInstance].value,
-                         NoSuchInstance())
+                         NoSuchInstance(''))
 
     def test_get(self):
         self.pdu_mock.oid_mapping[(1, 1)] = Mock()

virtualpdu/tests/unit/test_snmp_client.py

@@ -121,10 +121,10 @@ class TestSnmpClient(base.TestCase):
     def test_set_no_such_instance(self):
         oid = (1, 3, 6)
         self.command_generator_mock.setCmd.return_value = \
-            (None, 0, 0, [(oid, NoSuchInstance())])
+            (None, 0, 0, [(oid, NoSuchInstance(''))])
         value = self.snmp_client.set(oid, '43 thousands')
 
-        self.assertEqual(NoSuchInstance(), value)
+        self.assertEqual(NoSuchInstance(''), value)
 
         self.oneliner_cmdgen.UdpTransportTarget \
             .assert_called_with((sentinel.hostname, sentinel.port),