Make default params to CategoryViewer usable as defaults.

If the '$from' and '$to' parameters to the CategoryViewer constructor are left
off, they default to empty arrays. This makes later checks for $from['page'],
etc. issue an 'Undefined index' notice.

Using 'empty' would be more concise but could hide bugs.

Change-Id: I78793d13745d1fc1010b8e6861bdc3a89a39a87f