fix `to_dense` in `AbstractOperator`

Created by: PhilipVinc

(A minor change that I'll need for Lindblad)

The fact that there is only one entry per change is a silent assumption that holds true at the moment, but is not really enforced nor needed anywhere.

Moreover, with to_sparse you don't have this problem.

I came across this while working with Lindblad ME because there I have multiple entries with no changes.