Fix error on empty options chain

[stevenbischoff]

Fixes #1969

Error occurs when Yahoo has no calls or puts for a ticker. I wasn't sure whether y'all would prefer an exception in this case instead (first pull request!), but I chose to return an "empty" Options namedtuple since _download_options() returns an empty dict.

[ValueRaider]

I don't use options so no opinion. Try tagging the few people that use options (search here).

[BiggRanger]

Returning empty works perfectly for me. I'd rather check for nothing than have to handle an exception.