Create valid empty options

The HTML-spec[0] specifies:

> If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.

So a non-breaking space is valid solution.

[0] https://html.spec.whatwg.org/multipage/form-elements.html#the-option-element

inc/template.php

@@ -1645,7 +1645,7 @@ function tpl_mediaTree() {
  * @param string $empty empty option label
  * @param string $button submit button label
  */
-function tpl_actiondropdown($empty = '', $button = '>') {
+function tpl_actiondropdown($empty = ' ', $button = '>') {
     global $ID;
     global $REV;
     global $lang;