Bug 16426: follow-up of bug 15840 - correctly manage userid while inserting patrons

Bug 16426: follow-up of bug 15840 - correctly manage userid while inserting patrons

Bug 15840 tried to fix a bug but makes things more complicated than it
was before.
If an userid is not provided for 1 or more rows of the csv file, it
should not be updated. However, if a userid is provided and it already
used by an other patron, the import should fail for this row (but not
crash!).

Test plan:
0/ Create a patron with a userid=your_userid
1/ Use the import patron tool to update this userid
=> userid should have been updated
2/ Update another data and do not provide the userid
=> data should have been updated and not the userid
3/ Update another data and provide the userid, but set it to an empty
string, or '0'
=> data should have been updated and not the userid
4/ Update another patron, and set userid=your_userid
=> Update should fail and an error whouls be displayed ("already used by
another patron")

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>