Bug 15607: batch patron mod - do not update dates if not given

Bug 15607: batch patron mod - do not update dates if not given

Bug 15332 fixed a bug but introduced a bigger one. Even if dateenrolled and
dateexpiry are not modified, they will be updated to today.
Indeed, dt_from_string returns today without parameters.

Test plan:
- Set dateenrolled and expirydate for a patron
- Using the batch patron mod tool, update any field but dateenrolled and
expirydate.
=> Without this patch, the 2 date fields should be set to today
=> With this patch applied, the 2 date fields should not have been
modified.

Signed-off-by: Karam Qubsi <karamqubsi@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jesse Weaver <jweaver@bywatersolutions.com>