Handle nil values for Simplenote-synced content, tags, systemTags

If we receive nil for content, tags, or systemTags, replace it with an empty string, empty array, or empty array, respectively, rather than trying to store a nil in the NSDictionary.

My guess is that the server changed the way it was sending empty content, tags, and systemTags, and the old code didn't check the type before processing it.

Fixes #200