Calling JSON-RPC listTasks without a tenant throws an NPE before the request is sent: the MapStruct-generated ListTasksParamsMapperImpl.toProto calls setTenant(null) on the protobuf builder. This happens with both ListTasksParams.builder().build() and new ListTasksParams().

The fix adds the same conditionExpression null check already used for the other nullable fields in ListTasksParamsMapper, and for tenant in the neighboring request mappers (TaskQueryParamsMapper, TaskIdParamsMapper, etc.). Non-null tenants are still mapped as before.

REST and gRPC are not affected: both build ListTasksRequest manually and already guard the tenant.

Tests cover the default builder, the no-arg constructor and a non-null tenant, both at the mapper level and through JSONRPCTransport (checking the actual wire request). All 585 tests in the affected modules pass. The changed files pass strict Checkstyle; the 478 pre-existing violations on main are unchanged.

Related to #1207