Do not output nav menu class attribute if no classes exist

[niallkennedy]

Walker_Nav_Menu->start_el passes an array of class names and a string through class and id filters respectively. It's possible for a theme or plugin to empty out a class or id, overriding defaults.

The element string builder checks for an empty id attribute string value before outputting the id attribute. If a plugin empties the classes array then $classes_name will be similarly empty.

Patch applies the same conditional attribute output logic present in the id attribute to the class attribute as well.

[niallkennedy]

check if classes_name is an empty string before building an empty attribute for a nav menu walker element

[nacin]

Looks good. A note, strlen() is not necessary in either place.

[nacin]

In [20513]:

Only output the class attribute in wp_nav_menu() if we have classes to add. Don't waste time on an strlen() call. props niallkennedy, fixes #20483.