It will not break the log chain (which I think is your goal), but it won't affect shrinking the log file. In order to shrink the log file (not recommended) you need to have the log backed up (ie remove the copy_only).
I do not recommend doing a backup to null though. You will not be able to recover in the event of an emergency.
What I would recommend (if you do need to shrink the log) is to do a tlog backup and then shrink the log file. Shrinking the log file will affect performance of the system though both during and possibly after if the log needs to grow again.
So, using "with copy_only" does not backup the tlog?