[development] Two performance improvements

[Jakub Suchy]

Alan Dixon wrote:
> I've just submitted a simple bug report and proposed fix here:
> 
> http://drupal.org/node/74181
> 
> to resolve this issue. I hope someone can look at it.

Well, the patch is probably going to be rejected, but i don't think it's
a bad patch.
I think that "Do not query something that is empty" is a good idea.
Not because taxonomy_dhtml is poor written module, but because we want
to speed things up and save SQL queries. Once, forever.

Jakub Suchy