Fix *ISA = *glob_without_array

Fix *ISA = *glob_without_array

I broke this in 5.14 with commit 6624142a.

In trying to make *ISA = *Other::ISA work, I added logic to make
@Other::ISA’s existing magic now point to *ISA’s stash.  I skipped
that logic if *Other::ISA did not contain an array.  But in so
doing, I inadvertently skipped the call to mro_isa_changed_in at the
same time.