Bug 7660: Use first available categorycode when none passed in

Bug 7660: Use first available categorycode when none passed in

The code that populates the patron messaging preferences on initial form load
expects to have a category selected. Currently we only have one if one was
passed to the form. When creating an account from a parent, we don't have a
category explicitly selected - so we can just select the first of the possible
categories

To test:
 1 - In KTD set 'Juvenile' category to have some messaging preferences
 2 - Find a patron, say Edna Acosta, and 'Add guarantee'
 3 - In new form preferences are blank, cancel
 4 - Apply patch, restart all
 5 - Go to Edna, click 'Add guarantee'
 6 - Preferences are populated!
 7 - Cancel
 8 - Go to 'Patrons' module
 9 - Click "+ New patron"
10 - Confirm messaging preferences load correctly when not adding child

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>