Bug 21027: Fix checkout statistics if 1+ of the fields is not defined

Bug 21027: Fix checkout statistics if 1+ of the fields is not defined

TEST

- Use a patron who hasn't checked out items before
- Set  StatisticsFields to itype|ccode
- Check out some items without location
- Verify that statistics stable shows the correct number of checkouts
- Check out one item with a location or just change one of your checked out items
- Verify the total is still correct
- Change  StatisticsFields to location|itype|ccode
- Verify the total has changed now - it's only 1 (the one with a location)

Apply the patch and try again

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 17a71b990076efd04c15de6b3165c33fffe037a3)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>