isbn.py: convertIsbn10toIsbn13 should not fail for ISBN13

ISBN13 objects don't have a toISBN13 method, do not try to call that method
on ISBN13 objects.

The patch was originally introduced in a8008a210585759ac46f962eb2d6bd1ebad0460c
by Aadith1996, but reverted to fix minor issues.

Bug: T138911
Change-Id: I47118f6aa18cb659d1f9f96cdab93615e24af07b