Bug 21033: Remove few warns in acqui/basket.pl

Bug 21033: Remove few warns in acqui/basket.pl

Resolve (line numbers based on 16.11.x):
Use of uninitialized value in hash element at acqui/basket.pl line 337.
Use of uninitialized value in hash element at acqui/basket.pl line 338.
Use of uninitialized value in hash element at acqui/basket.pl line 340.
Use of uninitialized value in hash element at acqui/basket.pl line 342.
Use of uninitialized value in hash element at acqui/basket.pl line 344.

Argument "" isn't numeric in multiplication (*) at koha-tmpl/intranet-tmpl/prog/en/modules/acqui/basket.tt line 486.
Argument "" isn't numeric in multiplication (*) at koha-tmpl/intranet-tmpl/prog/en/modules/acqui/basket.tt line 591.

Test plan:
If you have older acq data, you may have records in aqorders with field
tax_rate_on_ordering is NULL. These orders will trigger the above warns.
If you do not have, you could simulate by setting this field to NULL.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>