1115527 - don't encode proxy url if not set

Addressing

Traceback (most recent call last):
  File "/usr/bin/yum", line 29, in <module>
    yummain.user_main(sys.argv[1:], exit_code=True)
  File "/usr/share/yum-cli/yummain.py", line 285, in user_main
    errcode = main(args)
  File "/usr/share/yum-cli/yummain.py", line 105, in main
    base.getOptionsConfig(args)
  File "/usr/share/yum-cli/cli.py", line 228, in getOptionsConfig
    self.conf
  File "/usr/lib/python2.6/site-packages/yum/__init__.py", line 891, in
<lambda>
    conf = property(fget=lambda self: self._getConfig(),
  File "/usr/lib/python2.6/site-packages/yum/__init__.py", line 362, in
_getConfig
    self.plugins.run('init')
  File "/usr/lib/python2.6/site-packages/yum/plugins.py", line 184, in
run
    func(conduitcls(self, self.base, conf, **kwargs))
  File "/usr/share/yum-plugins/rhnplugin.py", line 81, in init_hook
    proxy_url =
get_proxy_url(up2date_cfg).encode(locale.getpreferredencoding())
AttributeError: 'NoneType' object has no attribute 'encode'
Automatic commit of package [yum-rhn-plugin] release [2.2.5-1].