Dont use setSelectedValue in CategorySelector if there is no selected…

… category.

This patch fixes E_NOTICE: Undefined offset: 0 in
/.../lib/Zikula/Form/Plugin/CategorySelector.php Zeile 361

src/lib/Zikula/Form/Plugin/CategorySelector.php

@@ -357,7 +357,7 @@ public function loadValue(Zikula_Form_View $view, &$values)
                             $selectedValues[] = $c->getCategory()->getId();
                         }
                     }
-                    if ($this->selectionMode == 'single') {
+                    if ($this->selectionMode == 'single' && isset($selectedValues[0])) {
                         $this->setSelectedValue($selectedValues[0]);
                     } else {
                         $this->setSelectedValue($selectedValues);