Send a filter the server can read

Enabling background notifications failed with "Invalid filter". The
subscription asked to be notified about mail matching:

    filter: { inMailbox: null, notKeyword: "$seen" }

`inMailbox: null` meant "the inbox" in my head and nothing at all to
Stalwart, which needs a mailbox id there. It refused the whole
subscription, so the feature did not work at all for anyone who tried
it.

The Inbox's id is now passed in and used. Where it is not known the
condition is left out rather than sent empty: notifying more widely is a
worse default than filtering to the Inbox, but it is a working one, and
sending a malformed filter is not a fallback.

Two reasons this got out, both worth fixing rather than just the bug:

  - The tests checked the properties list and its ordering, and never
    looked at the filter. There is now one that walks every condition
    and fails on a null or undefined value, for both the known-inbox and
    unknown-inbox cases.

  - The mock accepted it happily, so nothing local disagreed with the
    code. It now refuses a filter condition with a null value and
    answers "Invalid filter.", which is what the live server said.
    Reproduced: the old payload is rejected, the new one accepted.

server/src/mock/index.ts

@@ -520,6 +520,18 @@ const handlers: Record<string, Handler> = {
         notCreated[cid] = { type: "invalidProperties", properties: ["url"], description: "Push endpoint must be https." };
         continue;
       }
+      // A filter condition with a null value is not a filter -- the real server
+      // answers "Invalid filter" and refuses the whole subscription. ihasmail
+      // shipped `inMailbox: null` meaning "the inbox", which meant nothing at
+      // all here, and the mock accepted it happily. It does not any more.
+      const badFilter = Object.entries((o.emailPush ?? {}) as Obj).find(([, cfg]) => {
+        const f = ((cfg as Obj)?.filter ?? {}) as Obj;
+        return Object.values(f).some((v) => v === null || v === undefined);
+      });
+      if (badFilter) {
+        notCreated[cid] = { type: "invalidArguments", properties: ["emailPush"], description: "Invalid filter." };
+        continue;
+      }
       // One per device: re-subscribing replaces rather than accumulates.
       const deviceId = String(o.deviceClientId ?? "");
       const clash = pushSubscriptions.findIndex((s) => s.deviceClientId === deviceId);