agent: Fix possible crash when modifying Configuration

[diconico07]

What this PR does / why we need it:
When modifying a Configuration that made the discovery handler drop the connection (e.g an invalid udev rule for udev handler), the finished_discovery_receiver gets None as the connection was dropped by the other end.
This lead to a crash of the agent as the Option was unwrapped here.

Now ignoring the None case to avoid crash.

Special notes for your reviewer:

If applicable:

• this PR has an associated PR with documentation in akri-docs
• this PR contains unit tests
• added code adheres to standard Rust formatting (cargo fmt)
• code builds properly (cargo build)
• code is free of common mistakes (cargo clippy)
• all Akri tests succeed (cargo test)
• inline documentation builds (cargo doc)
• all commits pass the DCO bot check by being signed off -- see the failing DCO check for instructions on how to retroactively sign commits

[kate-goldenring]

Thanks @diconico07!