Fix for Bug 6696, New category button broken when no categories defined

Fix for Bug 6696, New category button broken when no categories defined

The javascript tablesorter will trigger an error if we try
to apply it to a table with no <tbody>. That error prevented
the YUI button script to complete properly, hence the broken
button.

This patch hides the table in cases where there are no
categories and displays an error message instead.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
- Deleted all patron categories and checked button was broken
- Applied patch and checked again - button shows up
- Nice: There is a note now that you have to create categories

Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>