Permission.fromId is now @CheckForNull.

A user reported an exception likely caused by a permission defined in a now-disabled plugin:

```
java.lang.NullPointerException 
        at com.michelin.cio.hudson.plugins.rolestrategy.RoleBasedAuthorizationStrategy$ConverterImpl.marshal(RoleBasedAuthorizationStrategy.java:245)
```

(Line numbers are as in #1.)