autocomplete: fix empty autocomplete calls when toggling related tags.

This happens because toggling a related tag updates the tag box, which
triggers autocomplete, which can't find a tag because the cursor isn't
in the tag box, so it ends up making an autocomplete call for the empty
string.

app/javascript/src/javascripts/autocomplete.js.erb

@@ -199,6 +199,10 @@ Autocomplete.render_item = function(list, item) {
 };
 
 Autocomplete.autocomplete_source = function(query, type) {
+  if (query === "") {
+    return [];
+  }
+
   return $.getJSON("/autocomplete.json", {
     "search[query]": query,
     "search[type]": type,