Try
metadata_exists( 'post', $post->ID, 'display_sharing_buttons_save' )
This functions determine if meta-key exists and return true even with NULL value.
So in your case, if it returns FALSE you can show CHECKED by default.
Try
metadata_exists( 'post', $post->ID, 'display_sharing_buttons_save' )
This functions determine if meta-key exists and return true even with NULL value.
So in your case, if it returns FALSE you can show CHECKED by default.