By default, Dalli has two fallbacks if no server addresses are given:

- $MEMCACHE_SERVERS
- "127.0.0.1:11211"

However, MemCacheStore does its own check for addresses, and falls back
to "localhost:11211" if none are present.

This can lead to bugs in migrations from the deprecated :dalli_store
(provided by the Dalli) to :mem_cache_store:

```diff
-config.cache_store = :dalli_store     # could be implicitly relying on $MEMCACHE_SERVERS
+config.cache_store = :mem_cache_store # ignores $MEMCACHE_SERVERS
```

By removing our own fallback and simply passing `nil` to Dalli::Client,
we get its fallback logic for free. Tests are added so we can detect if
this ever changes.