Fix nil usage in dictionary warnings

Two warnings:
The first, |scriptPath| is being used in a dictionary and it can be nil:
   /Users/dougt/builds/Textual/Classes/IRC/IRCClient.m:2686:33: Dictionary value cannot be nil

The second, NSObjectIsEmpty(topic) (according to the compiler) can return true if topic is nil.
    /Users/dougt/builds/Textual/Classes/Views/Channel View/TVCLogController.m:1273:4: Value stored into 'NSMutableDictionary' cannot be nil