Can't list issues for a repository without any

Discovered by @abdu-almamou-sociomantic.

```
$ git hub issue list
Traceback (most recent call last):
  File "/home/luca/bin/git-hub", line 1683, in <module>
    main()
  File "/home/luca/bin/git-hub", line 1677, in main
    args.run(parser, args)
  File "/home/luca/bin/git-hub", line 578, in check_config_and_run
    cmd.run(parser, args)
  File "/home/luca/bin/git-hub", line 804, in run
    for issue in issues:
TypeError: 'NoneType' object is not iterable
```