Subtracting from a nonexistent set

Given an empty Redis database, the following works but fails with a `NoMethodError` with fakeredis:

```ruby
r = Redis.new
r.sadd("demo", "a")
r.sadd("demo", "b")
r.sdiff("nonexistent-key", "demo")
=> []
````

The exception raised by fakeredis:
```ruby
NoMethodError:
       undefined method `-' for nil:NilClass
````

The pull request fixes the problem so that it behaves the same as Redis. Thanks for merging/fixing it.