Fix unhandled exception in wallpaper widget
Fix unhandled exception in wallpaper widget

The lines:
```
        else:
            self.index += 1
            self.index %= len(self.images)
```
could result in a ZeroDivisionError if self.images is empty.

This commit makes the test to see if self.images is empty occurs
before this.

This could be responsible for the failing test in #2224
(unconfirmed) but would also be covered by #2236.

ramnes/qtile,ramnes/qtile,qtile/qtile,qtile/qtile