fix(federation): avoid empty body on signed requests

[benjaminfrueh]

Summary

Federated signed requests with empty body are sent with content-length: 0.
On some setups this header doesn't reach the receiving instance, which then rejects the requests with error missing content-length in header.

Solution

Send a minimal non-empty body {} instead of an empty string.

This is a limitation in core IncomingSignedRequest::verifyHeaders(), which requires the content-length header even when the body is empty. Working around it here because a core fix would need its own PR in nextcloud/server.

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