upgrade to drupal 5.4 broke something

Until using drupal 5.3, all worked fine. Upgrading to drupal 5.4, now something doesn't work as before.

I use TAC to give anonymous user a permission of "create" documents with a single category only. Before drupal 5.4, the only permitted category is automatically chosen. Now, with drupal 5.4 (taxonomy.module) an unuseful, new "Please choose" label is choosen. This is awful (if it's the only one, should be automatically chosen) and leads users to an unuseful, additional "click".

I think the new "feature" is in the taxonomy_form function of the taxonomy.module (this is from drupal 5.4 - drupal 5.5 and 5.6 again modified this...):
-----------
function taxonomy_form($vid, $value = 0, $help = NULL, $name = 'taxonomy') {
$vocabulary = taxonomy_get_vocabulary($vid);
$help = ($help) ? $help : $vocabulary->help;
$blank = 0;

if (!$vocabulary->multiple) {
$blank = ($vocabulary->required) ? t('- Please choose -') : t('- None selected -');
}
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Is the something we could do to avoid this in taxonomy access? maybe rewriting the "taxonomy_form" function?

Thanks