Dont snappy.inflate when is false

When server is off, memcached raises a `Memcached::ConnectionFailure` and return false.
Snappy.inflate was having troubles to inflate a `false`. So we should check if is `nil` OR `false` before inflating.

review @camilo @jduff 

this should fix:
```
Exception in Shopify: TypeError: no implicit conversion of false into String occurred 10 time(s):
```