Allowed null value for trial days on tier

refs TryGhost/Ghost@e26c977

- makes tiers validation to be more liberal by allowing setting `null` for `trial_days` in API

Author: rshbhgrg

packages/admin-api-schema/lib/schemas/tiers.json

@@ -42,7 +42,7 @@
           "type": ["number", "null"]
         },
         "trial_days": {
-          "type": "number"
+          "type": ["number", "null"]
         },
         "benefits": {
           "type": ["array", "null"],