dropdown.js: Avoid calling jQuery('#')

Calling jQuery('#') is considered a syntax error in jQuery 3.
This change avoids this issue, similar to the change in arrow.js
introduced in twbs/bootstrap#20019.

assets/javascripts/bootstrap/dropdown.js

@@ -29,7 +29,7 @@
       selector = selector && /#[A-Za-z]/.test(selector) && selector.replace(/.*(?=#[^\s]*$)/, '') // strip for ie7
     }
 
-    var $parent = selector && $(selector)
+    var $parent = $(selector === '#' ? [] : selector)
 
     return $parent && $parent.length ? $parent : $this.parent()
   }