Fix DNS zone serial number retrieval

In case of secondary DNS zones, the serial number
is not available until the zone completes a DNS transfer
from the master.

This patch addresses this issue by adding an extra check to
make sure the Microsoft_SOAType object exists before trying
to access it.

Change-Id: Id006a0aac6d0772b60c51d3a458a7b35aefe8dd3

os_win/tests/utils/dns/test_dnsutils.py

@@ -259,3 +259,13 @@ class DNSUtilsTestCase(test_base.OsWinBaseTestCase):
 
         self.assertIsNone(serial_number)
         mock_zone_exists.assert_called_once_with(mock.sentinel.zone_name)
+
+    @mock.patch.object(dnsutils.DNSUtils, 'zone_exists')
+    def test_get_zone_serial_zone_soatype_not_found(self, mock_zone_exists):
+        mock_zone_exists.return_value = True
+        self._dnsutils._dns_manager.MicrosoftDNS_SOAType.return_value = []
+
+        serial_number = self._dnsutils.get_zone_serial(mock.sentinel.zone_name)
+
+        self.assertIsNone(serial_number)
+        mock_zone_exists.assert_called_once_with(mock.sentinel.zone_name)

os_win/utils/dns/dnsutils.py

@@ -183,6 +183,8 @@ class DNSUtils(baseutils.BaseUtils):
 
         zone_soatype = self._dns_manager.MicrosoftDNS_SOAType(
             ContainerName=zone_name)
+        if not zone_soatype:
+            return None
         # Serial number of the SOA record
         SOA = zone_soatype[0].SerialNumber
         return int(SOA)