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_test.go

@@ -392,6 +392,27 @@ func TestLoadColumnWithDefaultExistingData(t *testing.T) {
 	}
 }
 
+// TestLoadColumnWithDefaultEmptyBytea verifies that an empty bytea ('\x', len 0)
+// is treated the same as NULL and returns the default value. Without this, the
+// postgres driver returns a non-nil empty slice that would reach the client
+// as a zero-byte ACK payload and crash the MHF gacha menu (see #175).
+func TestLoadColumnWithDefaultEmptyBytea(t *testing.T) {
+	repo, db, charID := setupCharRepo(t)
+
+	if _, err := db.Exec("UPDATE characters SET skin_hist=''::bytea WHERE id=$1", charID); err != nil {
+		t.Fatalf("Setup failed: %v", err)
+	}
+
+	defaultVal := []byte{0x00, 0x01, 0x02}
+	got, err := repo.LoadColumnWithDefault(charID, "skin_hist", defaultVal)
+	if err != nil {
+		t.Fatalf("LoadColumnWithDefault failed: %v", err)
+	}
+	if len(got) != 3 || got[0] != 0x00 || got[2] != 0x02 {
+		t.Errorf("Expected default value for empty bytea, got: %x", got)
+	}
+}
+
 func TestSetDeleted(t *testing.T) {
 	repo, db, charID := setupCharRepo(t)