[Fix #41763]: Redundant forward slash in SFTPToGCSOperator when destination_path is not specified or have default value

[Mayuresh16]

If the destination path is not specified, it defaults to an empty string. As a result, replacing base_path with an empty string is ineffective, causing the destination to retain the "/" prefix, if it has.

closes: #41763
related: #41763

[boring-cyborg]

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[Mayuresh16]

@potiuk @romsharon98 @moiseenkov Please merge the code if it looks good to you.

[boring-cyborg]

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