Fix empty tag field behavior when backspacing

If there were no tag chips, backspacing on the empty field would lose
focus (and on some browsers, another backspace would then go back).

tildes/static/js/behaviors/autocomplete-input.js

@@ -105,10 +105,12 @@ $.onmount('[data-js-autocomplete-input]', function() {
                     var $tagsHiddenInput = $("[data-js-autocomplete-hidden-input]");
                     var $lastChip = $chips.children().last();
 
-                    $(this).blur();
-                    if (!$lastChip.hasClass('active')) {
-                        $lastChip.addClass("active");
-                        $lastChip.children('[data-js-autocomplete-chip-clear]').first().focus();
+                    if ($lastChip.length) {
+                        $(this).blur();
+                        if (!$lastChip.hasClass('active')) {
+                            $lastChip.addClass("active");
+                            $lastChip.children('[data-js-autocomplete-chip-clear]').first().focus();
+                        }
                     }
                 }
                 break;