Bug 36277: Avoid useless warnings

Bug 36277: Avoid useless warnings

As mentioned on bug 36329, this endpoint is only used with one of this
parameters:

* item_type
* collection_code

The other will be NULL both on the DB and the (deserialized) request
body. For the data from the DB, the author added `|| q{}` but missed to
do so on the incoming parameters when generates the hash key.

This generates the following warnings when using from the UI:

[2024/03/15 11:42:51] [WARN] Use of uninitialized value in sprintf at /kohadevbox/koha/Koha/REST/V1/TransferLimits.pm line 146.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>