Bug 23083: Fix course reserve item editing if one of the value is set to UNCHANGED

Bug 23083: Fix course reserve item editing if one of the value is set to UNCHANGED

Test plan:
0: Do not apply the patches
1: Create an item with itype, ccode, location, and holding branch set
2: Create a course
3: Add item to course, changing the values for itype, ccode, and location
4: Click Edit for that item on course-details.pl
5: Assign new values for itype, ccode, and location but leave the holding library dropdown set to LEAVE UNCHANGED
6: Hit Save
7: Back on course-details.pl, observe that the item is still showing the values set in step 3
8: Apply this patch
9: Repeat 4 to 6
10: Back on course-details.pl, observe that the item is now showing the correct values
11: Edit the course reserve again, setting new values for itype, ccode, and location and setting the holding library dropdown to whatever the holding library for the item is currently
12: hit save
13: On course-details.pl, see that the change actually happened, the item is now showing the values set in step 9

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>