refactor(ff-admin): 优化余额计算逻辑

- 在获取余额时使用 NumberUtil.add 方法，确保结果不会为 null
- 避免潜在的 NullPointerException，提高代码健壮性

ff-admin/src/main/java/com/ff/common/service/impl/TenantGameQuotaServiceImpl.java

@@ -323,12 +323,12 @@ public class TenantGameQuotaServiceImpl implements ITenantGameQuotaService {
                     .tenantKey(gameBalanceExchange.getTenantKey())
                     .memberId(member.getId())
                     .build());
-            BigDecimal balanceOut = tenantGameQuotaFlowService.getExchangeMoneyByMemberId(TenantGameQuotaFlow.builder()
+            BigDecimal balanceOut = NumberUtil.add(tenantGameQuotaFlowService.getExchangeMoneyByMemberId(TenantGameQuotaFlow.builder()
                     .isOut(Boolean.FALSE)
                     .quotaType(quotaType)
                     .tenantKey(gameBalanceExchange.getTenantKey())
                     .memberId(member.getId())
-                    .build());
+                    .build()),BigDecimal.ZERO);
 
             // 初始化平台额度
             BigDecimal platformBalance = BigDecimal.ZERO;