Clear session observations that Zaptec sends without a value

[rhammen]

Zaptec ends a charging session by sending observations 721 (SessionIdentifier) and 722 (ChargerCurrentUserUuid) with no Value/ValueAsString field at all. state_to_attrs skips entries without a value, so the attributes kept the previous session's values indefinitely.

This explains the report in #200 (comment): charger_current_user_uuid appearing not to update with the RFID tag, while AuthenticationCode did. The attribute wasn't failing to update — it was never cleared, so it kept showing the previous session's token.

Map those two ids to an empty value instead of skipping them. Scoped to 721/722: Pulse is the only other observation seen without a value, and it never carries one.

Verified on a Zaptec Go 2 (fw 3.3.0.0): both observations appear valueless in the charger state after a session ends, and the derived attributes now clear.

Part of #200; the "Authorized by" sensors follow in a separate PR on top of this.

🤖 Generated with Claude Code

[rhammen]

Corrected the last line of the description: "within seconds of session end" was inferred from recorder history, which records when Home Assistant polled rather than when Zaptec sent the observation, so the timing was not something I actually measured. The valueless observations and the clearing behaviour are unchanged — only that claim was unsupported.