Update Constellix tests and fixtures for NULL SRV records

Thanks to @yzguy for assisting with API tests to confirm support

tests/fixtures/constellix-records.json

@@ -64,6 +64,62 @@
 	"roundRobinFailover": [],
 	"pools": [],
 	"poolsDetail": []
+}, {
+	"id": 1898527,
+	"type": "SRV",
+	"recordType": "srv",
+	"name": "_imap._tcp",
+	"recordOption": "roundRobin",
+	"noAnswer": false,
+	"note": "",
+	"ttl": 600,
+	"gtdRegion": 1,
+	"parentId": 123123,
+	"parent": "domain",
+	"source": "Domain",
+	"modifiedTs": 1565149714387,
+	"value": [{
+		"value": ".",
+		"priority": 0,
+		"weight": 0,
+		"port": 0,
+		"disableFlag": false
+	}],
+	"roundRobin": [{
+		"value": ".",
+		"priority": 0,
+		"weight": 0,
+		"port": 0,
+		"disableFlag": false
+	}]
+}, {
+	"id": 1898528,
+	"type": "SRV",
+	"recordType": "srv",
+	"name": "_pop3._tcp",
+	"recordOption": "roundRobin",
+	"noAnswer": false,
+	"note": "",
+	"ttl": 600,
+	"gtdRegion": 1,
+	"parentId": 123123,
+	"parent": "domain",
+	"source": "Domain",
+	"modifiedTs": 1565149714387,
+	"value": [{
+		"value": ".",
+		"priority": 0,
+		"weight": 0,
+		"port": 0,
+		"disableFlag": false
+	}],
+	"roundRobin": [{
+		"value": ".",
+		"priority": 0,
+		"weight": 0,
+		"port": 0,
+		"disableFlag": false
+	}]
 }, {
 	"id": 1808527,
 	"type": "SRV",

tests/test_octodns_provider_constellix.py

@@ -101,14 +101,14 @@ class TestConstellixProvider(TestCase):
 
                 zone = Zone('unit.tests.', [])
                 provider.populate(zone)
-                self.assertEquals(14, len(zone.records))
+                self.assertEquals(16, len(zone.records))
                 changes = self.expected.changes(zone, provider)
                 self.assertEquals(0, len(changes))
 
         # 2nd populate makes no network calls/all from cache
         again = Zone('unit.tests.', [])
         provider.populate(again)
-        self.assertEquals(14, len(again.records))
+        self.assertEquals(16, len(again.records))
 
         # bust the cache
         del provider._zone_records[zone.name]
@@ -163,7 +163,7 @@ class TestConstellixProvider(TestCase):
             }),
         ])
 
-        self.assertEquals(17, provider._client._request.call_count)
+        self.assertEquals(19, provider._client._request.call_count)
 
         provider._client._request.reset_mock()