Add null check on subscription blocks

`env.Content.subscriptionBlocks` is nullable and needs to be checked, otherwise we will get this exception:

```nodejs
error: TypeError: /home/thont/Desktop/habitrpg/views/shared/modals/members.jade:78
    76|       .panel-body
    77|         .form-group
  > 78|           each block in env.Content.subscriptionBlocks
    79|             .radio
    80|               label
    81|                 input(type="radio", name="subRadio", value="#{block.months}", ng-model='gift.subscription.months')

Cannot read property 'length' of undefined
```