getBlogForDotComBlogId now returns null if the blog id was found in db

fix #1040 

We should eventually use another kind of OAuthAuthenticator https://github.com/wordpress-mobile/WordPress-Android/blob/c247eca4b7e7c97b00809ff5f277cb0bed773e24/src/org/wordpress/android/networking/OAuthAuthenticator.java?source=c#L18 for this kind of reader related requests that apply to not owned blogs (like, follow, ...)