xml: T5738: revert invalid change from lower character limit - 0 length must be allowed

This reverts a change from commit a72ededa0 ("xml: T5738: lower maximum
description to 255 characters") which incresaed the lower limit from 0 to 1.

We actually require 0 length value for description nodes as introduced in
commit 6eea12512e ("xml: T1579: allow zero length for description").