fix(api): return [] not null for empty leaderboard and account points

[Ryanmello07]

A nil slice marshals as JSON null. The gomobile sdk binds these fields as pointers, so null becomes a nil object that the Android client dereferences inside a JNI callback — the NPE cannot cross JNI and ART aborts the whole process:

JNI DETECTED ERROR IN APPLICATION: CallStaticVoidMethod called with pending exception
java.lang.NullPointerException: ...LeaderboardEarnersList.len() on a null object reference

Confirmed on device: opening the Leaderboard tab hard-crashes the app. This hits any deployment whose leaderboard query returns zero rows — a new or custom server, or one before its first payout cycle.

Reproduced live before the fix, and verified after:

before:  /stats/leaderboard -> {"earners":null}
         /account/points    -> {"account_points":null,"network_points":null}
after:   /stats/leaderboard -> {"earners":[]}
         /account/points    -> {"account_points":[],"network_points":[]}

Three instances

1. model.GetLeaderboard — earners is a named return that is only ever appended to, so zero rows leaves it nil.
2. controller.GetLeaderboard error path — returns a result with only Error set, leaving Earners nil, so a transient query error crashes the app exactly as an empty leaderboard does.
3. model.FetchAccountPoints — identical shape, feeding both network_points and its dual-emitted account_points alias, so /account/points returned two null fields.

Not affected

GetNetworkLeaderboardRanking returns a NetworkRanking value, not a pointer, so a network with no payouts already serialises as an object. A nil *NetworkRanking on the client is the binding reflecting the JSON and resolves with the above.

Audit of the whole class

Every function in model/ and controller/ with a named slice return that is only appended to: six matches, of which only the two above are API-facing. listClientReliabilityPartitions and MaintainClientReliabilityPartitions are internal maintenance, GetProxyIdsSince has no controller caller, and GetCircleUCUsers feeds internal processing.

Tests

DB-backed, calling the real functions against an empty database and asserting on the marshalled bytes rather than on non-nilness — [] versus null is the property that actually reaches the client. Teeth-checked: removing either initialisation fails them with "returned a nil slice; it marshals as null and crashes the client".

Note: TestAccountPoints in the controller package fails, but fails identically on the parent commit (same panic at account_point_controller_test.go:16) — pre-existing and unrelated to this change.

[Ryanmello07]

Superseded by #425, which consolidates this stack into a single branch rebased onto current main. The work is unchanged and included there — this PR's commits are in #425's history (or, where the chain rewrote them, their final form is). Closing to keep review in one place.