account_invoice_merge : little fixes

Hello,

I make a litte fix in account_invoice_merge.

uos_id is not a required field on invoice line so I refactored the part where it is use.

I also add a context key (is_merge) to disting invoice from merge invoice. It can be usefull to know this info at invoice creation (in my case, at creation I add some line function of the product, I don't want to get these line duplicate).
And finally I pass, this context to the create method.
@pedrobaeza as I try to explain in the first message, I add this context key to distinguish normal invoice creation from merge invoice creation. In my use case, at creation I add some line function of the product, I don't want to get these line duplicate.

@sbidoul I thought it can be usefull for other. But it is not a problem to move this part in my customer module.