MDL-78953 autocomplete: Add only one empty option when deselecting item.

Each option that was deselected was prepending an empty option in the select.

lib/amd/src/form-autocomplete.js

@@ -240,7 +240,9 @@ define([
         var selectedItemValue = $(item).attr('data-value');
 
         // Preprend an empty option to the select list to avoid having a default selected option.
-        originalSelect.prepend($('<option>'));
+        if (originalSelect.find('option').first().attr('value') !== undefined) {
+            originalSelect.prepend($('<option>'));
+        }
 
         // Look for a match, and toggle the selected property if there is a match.
         originalSelect.children('option').each(function(index, ele) {