fixes#8310
test plan:
* load some sis data for terms/courses/sections/enrollments into an
account
* do a batch mode import of an enrollments.csv with just the headers,
no data
* all enrollments should be deleted, but courses and sections should
be left alone
Change-Id: I05beb577bfb4573e97f8b398aeb4bfff5df78a4f
Reviewed-on: https://gerrit.instructure.com/10265
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
8beb4e0fd4