oracleobjectstorage: fix: upload empty streams without multipart - #9903

What does this change do?

Fix the Oracle Object Storage FsPutStream/0 integration-test failure.

OCI rejects completion of a multipart upload with no parts. For an unknown-size stream, probe through a buffered reader; if it is already at EOF, use a regular upload instead. Peek preserves non-empty stream data for the selected upload path.

The separate companion PR #9902 fixes the Oracle Object Storage FsEncoding failures. Together, these two trivial fixes address the remaining failures from the backend integration suite.

Linked issue

No issue - trivial backend bug fix.

For new or changed backends

Tested against a configured Oracle Object Storage remote:

go test -v -run '^TestIsZeroLength$' ./backend/oracleobjectstorage
go test -v -run 'TestIntegration/FsMkdir/FsPutFiles/FsPutStream' ./backend/oracleobjectstorage

The focused unit test passes all isZeroLength paths. The integration test passes both zero-byte and 100-byte streamed uploads.

A full backend run on this branch still has the separate FsEncoding failure, addressed by #9902.

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines (https://github.com/rclone/rclone/blob/master/CONTRIBUTING.md#submitting-a-new-feature-or-bug-fix).
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance (https://github.com/rclone/rclone/blob/master/CONTRIBUTING.md#ai-assisted-contributions), and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style (https://github.com/rclone/rclone/blob/master/CONTRIBUTING.md#commit-messages).
• (Backend changes only) test_all passes for this backend and if submitting a new backend can provide a test account for the integration tester - see CONTRIBUTING.md (https://github.com/rclone/rclone/blob/master/CONTRIBUTING.md#integration-tests).
• This Pull Request is ready for review.