Port 68516b0b2fa67112ad1ddc2bfc69dbd25c1bf5df to master:

manage_config_email_page: handle error case for
 array_merge_recursive2

The array_merge_recursive2() function within
manage_config_email_page.php was not performing a check to ensure that
both arrays contain an element with a specified key. This could lead to
warnings and errors being produced when the function is called.