fix(services/seafile): avoid defaulting list size to zero

[Xuanwo]

Which issue does this PR close?

Part of #7062.

Rationale for this change

seafile lister currently maps missing size in list response to 0.
This mixes "size is unknown in list response" with "real zero-byte file", which is semantically incorrect.

What changes are included in this PR?

• Update core/services/seafile/src/lister.rs:
  • Remove unwrap_or(0) when setting file content_length.
  • Set content_length only when info.size exists.

This preserves correct zero-byte semantics while allowing core completion logic to fill missing sizes when needed.

Validation:

• cargo fmt
• cargo check --features services-seafile,tests
• cargo test -p opendal-service-seafile

Are there any user-facing changes?

Yes.

For seafile, list entries no longer report content_length = 0 when list response omits size.

AI Usage Statement

This PR was developed with Codex (GPT-5) assistance.