Bug 37263: Fix handling of 'All' values on 'default article request fees'

Bug 37263: Fix handling of 'All' values on 'default article request fees'

To test:
1. Enable ArticleRequests sys pref
2. Visit <staff_url>/cgi-bin/koha/admin/smart-rules.pl
3. Under 'Default article request fees' on 'Patron category' select 'All' and input a numeric Free
4. Click 'Add'. Notice it blows up.
5. Apply patch. Repeat.

Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>
Signed-off-by: Emmi Takkinen <emmi.takkinen@koha-suomi.fi>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 84a31260ad8f875beb20ca83d2889ddd5503855f)