warning with join in nav-menu-template.php (L 98) when apply_filters nav_menu_css_class return null

[eadoking]

there is a warning because apply_filters on nav_menu_css_class return null and a warning happens in join clause , file "nav-menu-template.php" line 98

[briandichiara]

patch for nav-menu-template.php, resolves notices when non-array passed through filter

[markjaquith]

In 30032:

Cast class names to an array to avoid a potential warning.

props briandichiara
fixes #29743

[SergeyBiryukov]

there is a warning because apply_filters on nav_menu_css_class return null

I think this should be considered a developer error. If the goal is to remove the classes, __return_empty_array() should be used here instead of __return_null.

But if we're going to keep [30032], we should do the same for body_class, post_class, comment_class, and page_css_class filters as well.

[johnbillion]

I would like to revert r30032. I agree with Sergey that this is a developer error and the warning is appropriate.

[johnbillion]

Fixed in r30592.