Discussion settings: save 'closed' instead of empty string when pingback/comment checkboxes are unchecked

When the 'Allow link notifications' or 'Allow comments' checkboxes are
unchecked in Settings > Discussion, the form submits without those fields.
The generic handler in options.php saves null, which update_option stores
as an empty string.

This causes problems because:
- The REST API schema only accepts 'open' or 'closed' for ping_status
  and comment_status, rejecting empty strings with a 400 error
- Gutenberg's Real-Time Collaboration feature round-trips the full entity
  record through REST on page load, exposing this as a visible error
- MySQL strict mode configurations crash on NULL for NOT NULL columns

The fix adds hidden inputs before each checkbox so that unchecking submits
'closed' instead of nothing. When checked, the checkbox value 'open'
overrides the hidden input. This is the standard HTML pattern used
elsewhere in WordPress.

Author: chubes4

src/wp-admin/options-discussion.php

@@ -52,10 +52,12 @@
 <?php _e( 'Attempt to notify any blogs linked to from the post' ); ?></label>
 <br />
 <label for="default_ping_status">
+<input type="hidden" name="default_ping_status" value="closed" />
 <input name="default_ping_status" type="checkbox" id="default_ping_status" value="open" <?php checked( 'open', get_option( 'default_ping_status' ) ); ?> />
 <?php _e( 'Allow link notifications from other blogs (pingbacks and trackbacks) on new posts' ); ?></label>
 <br />
 <label for="default_comment_status">
+<input type="hidden" name="default_comment_status" value="closed" />
 <input name="default_comment_status" type="checkbox" id="default_comment_status" value="open" <?php checked( 'open', get_option( 'default_comment_status' ) ); ?> />
 <?php _e( 'Allow people to submit comments on new posts' ); ?></label>
 <br />