Handling absolute number zero and null differently

Tvdb returns an empty string for unknown absolute numbers, allowing us to distinguish between unknown and zero.
I've been told the tvdb doesn't contain zero as absolute number, but they do exist. Like https://trello.com/c/BI30zYEB/817-parser-with-0-as-absolute-number-causes-error-in-episoderepository.

Instead of just ignoring FindEpisode zero. I instead changed the code to store null in the database.

@markus101 You're feedback is particularly important, you might've had a valid reason to store zero instead of null (despite that the model has a Nullable<Int32> property.)

I've also moved the error handling @kayone added to catch the exception to the GetTrackedDownload method.

"Can not search for an episode without an absolute episode number" sounds a bit better (add the an).