[CAMEL-17204] Fix warning message for S3 multipart uploads

[joeoconnor-toast]

Don't log a warning if this is the first part of a multipart upload.

If this is the first part of the file, then filePosition will be zero and we will attempt to skip zero bytes. This change avoids the call to skip entirely for the first part.

https://issues.apache.org/jira/browse/CAMEL-17204

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