Fix checking for config.proxy_username

[yhekma]

When using basic authentication for the url to fetch and using a proxy without authentication, all the authentication gets stripped away when the proxy_username exists in the config file /etc/pulp/server/plugins.conf/yum_importer.json but is empty. This is because in threaded.py the code checks if proxy_username == None, which is untrue (it is an empty string). Since

variable = ''
if variable:
  do_something

evaluates works as well, this is the better option in my opinion.

[bowlofeggs]

I think this makes sense, I doubt any proxy server allows the empty string as a username. Thanks for your contribution!

[bowlofeggs]

I added a unit test for this PR in #48

[bowlofeggs]

I filed a ticket about this issue: https://pulp.plan.io/issues/1820