fix(handlers): harden gacha_items and boost time limit ACK paths

Both bugs surfaced when running protbot against a live Erupe instance.

LoadColumnWithDefault now treats an empty bytea ('\x', len 0) the same
as NULL. The postgres driver returns a non-nil empty slice for empty
bytea, so the prior `data == nil` check let a zero-byte slice reach
handleMsgMhfReceiveGachaItem, which forwarded it to the client as a
malformed gacha_items response. The MHF client interprets a zero-byte
count field as a protocol error and crashes the gacha menu, matching
the #175 symptom class. The handler also gains a defensive fallback
for len(data) == 0 in case another caller hits the same edge.

handleMsgMhfGetBoostTimeLimit was sending two ACKs for a single
request: doAckBufSucceed with the real payload, then an unconditional
doAckSimpleSucceed on the same ack handle. The second ACK was dead on
arrival (the handle was already consumed) but is a latent protocol
bug. Drop it and update the regression test that was asserting the
buggy 2-packet behavior.

server/channelserver/repo_character.go

@@ -167,7 +167,11 @@ func (r *CharacterRepository) LoadColumnWithDefault(charID uint32, column string
 	if err != nil {
 		return defaultVal, err
 	}
-	if data == nil {
+	// Treat empty bytea ('\x', len 0) the same as NULL. The postgres driver
+	// returns a non-nil empty slice for empty bytea, so a bare `data == nil`
+	// check would send zero bytes to the client — which the MHF client
+	// interprets as a malformed response and crashes on (see #175).
+	if len(data) == 0 {
 		return defaultVal, nil
 	}
 	return data, nil