archrelease: Fix SVN error when repository is empty

If trash is empty, `svn rm -q "${trash[@]}"` fails with:

    svn: Try 'svn help' for more info and then
    svn: Not enough arguments provided

This doesn't result in any further malfunction but is kind of confusing
and unexpected. Skipping `svn rm` on an empty trash fixes this.

Reported-by: Alexander Rødseth <rodseth@gmail.com>
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>