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`.

Start by locating the definitions and existing tests for Connection and ScopedConnection, then inspect how default-constructed instances are assigned and compared. Done means assigning or comparing either type with nullptr has the same behavior as assigning or comparing with a default-constructed connection, with tests covering both types.

Written by the indexing model from the issue text.