Fix Torquebox::Cache when Infinispan is not available

After upgrading to TorqueBox 3.0.0, I kept getting the following error in development:

```
NoMethodError (undefined method `encode' for nil:NilClass)
/torquebox-cache-3.0.0-java/lib/torquebox/cache.rb:260:in `encode'
/torquebox-cache-3.0.0-java/lib/torquebox/cache.rb:163:in `get'
/torquebox-cache-3.0.0-java/lib/active_support/cache/torque_box_store.rb:100:in `read_entry'
```
It turns out this was caused by not running in TorqueBox, and hence Infinispan was not available. When this happens, no instance variables are ever set and none of the methods that rely on `@codec` (such as `#encode`) work either.