Bug fix GCSToS3Operator: avoid ValueError when replace=False with files already in S3

[Adaverse]

When few or all files that are already there in S3 that needs to be transferred from GCS to S3 with replace=False and s3_dest_url not ending with / ends up in the below error -

The reason being the filter snippet below doesn't filter the already existing filter files leading to the error due to missing /

# remove the prefix for the existing files to allow the match
existing_files = [file.replace(prefix, "", 1) for file in existing_files] # <---- prefix is missing /
files = list(set(files) - set(existing_files))

The tests do not catch it because this scenario is not covered.

Hence have performed the below tasks to mitigate the above scenario -

• Apply / if not present (avoiding if prefix is empty)
• Increase test coverage to include above scenaio

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.