[ADD] partner_firstname for v8

because it's working as is
On 07/29/2014 04:42 PM, Pedro M. Baeza wrote:
>
> Do it also always, having or not records. Maybe you can check if the 
> restriction is already applied, but this way, you will always get 
> message when there are records.
>

Just ported from v7 version of the module, but you are right.
Modified it and used the query
```
UPDATE res_partner SET lastname = name WHERE name IS NOT NULL AND 
lastname IS NULL
```
to be sure to never overwrite lastname