feat: skip file info transfer for /dev/null placeholder

[abakum]

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[abakum]

feat: skip file info transfer for /dev/null placeholder

Problem: In WebDAV/TCP forwarding mode, real files are transferred outside of croc — through a WebDAV server or TCP forwarding. However, croc's Send() method is the single entry point that performs several critical functions simultaneously:

1. PAKE key exchange — establishing a shared secret key
2. Channel encryption — creating an encrypted tunnel
3. File metadata transfer — information about files being sent
4. Actual data transfer — binary file stream

For tunnel-only use cases, only steps 1-3 are needed — no real file data should be sent through croc.

Solution: When the sender has exactly one file and it is /dev/null, skip the file info transfer by setting Step2FileInfoTransferred = true and returning early. The /dev/null placeholder:

• Exists in the filesystem — no errors when opening
• Has zero size — transfer completes instantly
• Contains no data — nothing extra is sent

Analogy: Like sending an empty envelope through a courier service so that:

• The courier (croc) establishes a secure route (encrypted channel)
• The recipient confirms their identity (PAKE authentication)
• After that, the real cargo travels via a separate highway (WebDAV/TCP forwarding)

This enables reusing croc's secure connection infrastructure (PAKE + encryption + authentication) as a transport tunnel for WebDAV or any other application, without sending any real files through the croc protocol itself.