Bug 29349: Do not assume holding branch is a valid pickup location

Bug 29349: Do not assume holding branch is a valid pickup location

The original code for pickup locations when placing item-level holds
picked the currently logged-in library.

We made things more robust, as the logged-in library might not be a
valid pickup location for the patron and item. But it was wrongly chosen
to use the holding branch as the default.

A more robust approach is needed, and this precedence is picked this
time (it could be configuration-driven in the future):

    - Logged-in library
    - Empty

To test:
1. Pick a biblio with various valid pickup locations, some not including
   the logged-in library.
2. Pick a patron for placing the hold
=> FAIL: Notice that (when valid pickup location) the holding branch is
always chosen
3. Apply this patch
4. Repeat 2
=> SUCCESS: If valid pickup location, the logged-in branch is picked as
default for item-type level. When it is not, an empty dropdown is used
as a fallback.
5. Sign off :-D

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>