Fix #45: assert.Empty should work with channels

This adds the channel case to `isEmpty()`. The error message you would get in your code is a little bit cryptic when using `assert.Empty` with a channel, so if you have ideas on how to make it better, let me know:

    Should be empty, but was 0x2083be160

To clarify: this is because channels don't have a default format for printing as opposed to every other type. Maybe something like this could work:

    Should be empty, but was len(0x2083be160) = 1