fix(gacha): validate reward pool before charging currency (#175)

Filter out gacha entries with no items at the query level (EXISTS
subquery) and reorder Play methods to validate the pool before
calling transact(), so players are never charged for a misconfigured
gacha.

server/channelserver/svc_gacha_test.go

@@ -27,6 +27,7 @@ func TestGachaService_PlayNormalGacha(t *testing.T) {
 	}{
 		{
 			name:    "transact error",
+			pool:    []GachaEntry{{ID: 10, Weight: 100}},
 			txErr:   errors.New("tx fail"),
 			wantErr: true,
 		},
@@ -36,6 +37,10 @@ func TestGachaService_PlayNormalGacha(t *testing.T) {
 			poolErr: errors.New("pool fail"),
 			wantErr: true,
 		},
+		{
+			name:    "empty reward pool",
+			wantErr: true,
+		},
 		{
 			name:    "success single roll",
 			txRolls: 1,
@@ -95,6 +100,7 @@ func TestGachaService_PlayStepupGacha(t *testing.T) {
 	}{
 		{
 			name:    "transact error",
+			pool:    []GachaEntry{{ID: 10, Weight: 100}},
 			txErr:   errors.New("tx fail"),
 			wantErr: true,
 		},
@@ -104,6 +110,10 @@ func TestGachaService_PlayStepupGacha(t *testing.T) {
 			poolErr: errors.New("pool fail"),
 			wantErr: true,
 		},
+		{
+			name:    "empty reward pool",
+			wantErr: true,
+		},
 		{
 			name:    "success with guaranteed",
 			txRolls: 1,