Fix optional values for upload field (#2746)
Fix optional values for upload field (#2746)

*Some* fields, like the upload field, return false when the data
is deleted instead of an empty array. The patch introduced in #2744 did
not account for that: It removes the fast return only for empty
variables but not for non-arrays.

In 2.6.x, both empty values and non-arrays would still issue the delete,
hence we need to continue. We can ignore the passed value and just
continue with an empty array.

Re #2744
Fixes #2733 properly
Picked from 	f228874
bauhouse/sym-todo-list,bauhouse/sym-todo-list,michael-e/symphony-2,symphonycms/symphony-2,symphonycms/symphony-2,michael-e/symphony-2