fix: skip blank lines in rss feed file

[kushals256]

fixes #920

blank lines in the rss feeds file produced an empty feed url. urlparse gave an empty netloc so the bot tried to open the cache directory as a file and crashed.

skip blank lines when reading the feed file. added a small helper and unit tests.

[zulipbot]

@kushals256 You've referenced #920 in your pull request description, but you have not referenced it in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #920..

An example of a correctly-formatted commit:

commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51.

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