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.

Fixes #200
Merge pull request #201 from jtamboli/systemtags-sync-fix

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