Bounty 389 undefined method banner

**Bug:** https://assembly.com/coderwall/bounties/389#
**Description:** Some users a getting "undefined method banner for nil:NilClass" errors

**Cause:** when a use is not logged in, `current_user` returns nil, thus causing the `NoMethodError` when `user.banner` gets called from within `location_image_url`.

**Fix:** removed the else block, forcing to return nil when other conditions are not satisfied.  Returning nil is safe, because the calling `image_path` returns '/assets' when a nil argument is passed.