Fix confusing CHECK-EMPTY in affine-map test

This commit replaces // CHECK-EMPTY because it is an extremely confusing way of
allowing (but not checking for) empty lines. The problem is that // CHECK-EMPTY
is **only a comment** and does not do anything.

I originally tried to use // CHECK-EMPTY: but errors occured due to missing
newlines.

The intended behavior of the test is to enforce nothing (not even a newline)
is printed and the proper way to check for this is to use CHECK-NOT.

Thanks to @rxwei for helping me figure out to use CHECK-NOT properly.

PiperOrigin-RevId: 210286262