[FIX] payment_sips: special characters

The field `transactionReference` of Sips must contain only
alphanumerical characters. Previous commit 31dc6b88 solved
the special case of several attempts to pay, but a much simpler case may
appear in v11. Paying an invoice is likely to fail since the reference
contains by default `/`.

In the previous commit, we could simply modify the common `payment`
module code. In this specific case, however, we only change the
reference if the acquirer is Sips.

opw-1841483