Update Mythic Beasts tests and fixtures for NULL SRV records

Thanks to @pwaring for assisting with API tests to confirm support,
and reaching out to Mythic Beasts to obtain a fix

tests/fixtures/mythicbeasts-list.txt

@@ -5,6 +5,8 @@
 @ 3600 SSHFP 1 1 bf6b6825d2977c511a475bbefb88aad54a92ac73
 @ 3600 SSHFP 1 1 7491973e5f8b39d5327cd4e08bc81b05f7710b49
 @ 3600 CAA 0 issue ca.unit.tests
+_imap._tcp 600 SRV 0 0 0 .
+_pop3._tcp 600 SRV 0 0 0 .
 _srv._tcp 600 SRV 10 20 30 foo-1.unit.tests.
 _srv._tcp 600 SRV 12 20 30 foo-2.unit.tests.
 aaaa 600 AAAA 2601:644:500:e210:62f8:1dff:feb8:947a

tests/test_octodns_provider_mythicbeasts.py

@@ -378,8 +378,8 @@ class TestMythicBeastsProvider(TestCase):
             zone = Zone('unit.tests.', [])
             provider.populate(zone)
 
-            self.assertEquals(15, len(zone.records))
-            self.assertEquals(15, len(self.expected.records))
+            self.assertEquals(17, len(zone.records))
+            self.assertEquals(17, len(self.expected.records))
             changes = self.expected.changes(zone, provider)
             self.assertEquals(0, len(changes))
 
@@ -445,7 +445,7 @@ class TestMythicBeastsProvider(TestCase):
                                       if isinstance(c, Update)]))
             self.assertEquals(1, len([c for c in plan.changes
                                       if isinstance(c, Delete)]))
-            self.assertEquals(14, len([c for c in plan.changes
+            self.assertEquals(16, len([c for c in plan.changes
                                        if isinstance(c, Create)]))
-            self.assertEquals(16, provider.apply(plan))
+            self.assertEquals(18, provider.apply(plan))
             self.assertTrue(plan.exists)