Add nullptr semantics to Connection and ScopedConnection

The c++ standard library uses `nullptr` to mean "empty" not only for pointers (e.g., `std::function`). I suggest to add the same semantics to `Connection` and `ScopedConnection`, i.e., assigning and comparing with `nullptr` has the same effect as assigning and comparing with a default constructed `[Scoped]Connection`.

首先定位 Connection 和 ScopedConnection 的定义及现有测试，然后检查默认构造的实例是如何进行赋值和比较的。当对任一类型赋值 nullptr 或将其与 nullptr 比较时，其行为与对默认构造的连接进行赋值或比较相同，并且有覆盖这两种类型的测试时，即视为完成。

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