Consider not adding the "Automatic Updates" column when plugin_status is 'mustuse' or 'dropin'

[pbiron]

WP_PLugins_List_Table doesn't add row actions for plugins when these statuses are current.

Especially for dropins, these are usually not plugins that are "updateable", e.g.:

• install.php
• db.php (Query Monitor symlinks this to a file within it's plugin dir
• advanced-cache.php, usually added by caching plugins
• etc

I guess it's possible that mustuse plugins could be "updateable", but again, they are usually symlinks to files within other plugins (.e.g, ManageWP Worker.

At the very least, wp_autoupdates_add_plugins_autoupdates_column_content() should do a check such as:

if ( ! isset( $plugins[ $plugin_file ] ) ) {
	return;
}

otherwise PHP notices (Undefined index) are generated when constructing $aria_label, since $plugins[ $plugin_file ] isn't set.