Fixed awkward title when subtitle is an empty string

[araishikeiwai]

Sometimes, the subtitle given by the Google Books API is an empty string (e.g. https://www.googleapis.com/books/v1/volumes?q=isbn:9786029225938).

This PR tries to fix the awkward resulting title due to no empty string checking when building the title.

Before: item.title would yield "And The Mountains Echoed: " After: item.title would yield "And The Mountains Echoed"