Fix nil deserialization. Fixes issue #115.

I have no idea what I'm doing. Success!
From these two lines, you can see that this change does not introduce any regressions:

1. The method exits early if the argument is `nil` (i.e., it doesn't respond to `has_key?`).
2. In the line below, the code expects the argument to respond to `has_key?`, which would raise an exception if it didn't. This is the exception that I am seeing in our code. I doubt that raising an `NoMethodError` is something that anyone would depend on.