fix GCSToGCSOperator bug when copy single object with replace to False

[NatchapolLaow]

This PR aims to fix a small bug in GCSToGCSOperator when copy single object with replace to False.

Currently, when replace set to False, the set operation is used to check replacing file(s).
It works well except in line 412 of _copy_source_without_wildcard function that check objects is a single object. An error occurred while this if statement tries to get element by index from set object.
if len(objects) == 1 and objects[0][-1] != "/":
I simply convert back to a list. It will not affect other downstream because other objects are a list too.

I also attached an error log that I found this bug last month.

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.