matrix-sdk depends on never type fallback

[jplatte]

I'm getting a future-compat warning locally. The full thing (w/o color):

The package `matrix-sdk v0.12.0 (/home/jplatte/code/matrix/matrix-rust-sdk/crates/matrix-sdk)` currently triggers the following future incompatibility lints:
> warning: this function depends on never type fallback being `()`
>    --> crates/matrix-sdk/src/event_handler/mod.rs:900:11
>     |
> 900 |     async fn test_add_room_event_handler() -> crate::Result<()> {
>     |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>     |
>     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
>     = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/never-type-fallback.html>
>     = help: specify the types explicitly
> note: in edition 2024, the requirement `!: event_handler::EventHandlerResult` will fail
>    --> crates/matrix-sdk/src/event_handler/mod.rs:935:50
>     |
> 935 |           client.add_room_event_handler(room_id_b, move |_ev: OriginalSyncRoomNameEvent| async {
>     |  __________________________________________________^
> 936 | |             unreachable!("No room event in room B")
> 937 | |         });
>     | |_________^
>
> warning: this function depends on never type fallback being `()`
>    --> crates/matrix-sdk/src/event_handler/mod.rs:976:11
>     |
> 976 |     async fn test_remove_event_handler() -> crate::Result<()> {
>     |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>     |
>     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
>     = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/never-type-fallback.html>
>     = help: specify the types explicitly
> note: in edition 2024, the requirement `!: event_handler::EventHandlerResult` will fail
>    --> crates/matrix-sdk/src/event_handler/mod.rs:988:49
>     |
> 988 |           let handle_a = client.add_event_handler(move |_ev: OriginalSyncRoomMemberEvent| async {
>     |  _________________________________________________^
> 989 | |             panic!("handler should have been removed");
> 990 | |         });
>     | |_________^
>

We'll probably want to add () expressions there, even though it's pretty ugly.