DSLX: Bad error on `send` on channel array (instead of a channel)

I accidentally did:
```
my_chans: chan[4];
...
let tok = send(tok, my_chans, foo);
```

Obv. wrong: you send on individual channels, not channel arrays (tho that's an interesting idea). Instead of a nice error message, I got a XLS_RET_CHECK failure: `Check failed: ::absl::OkStatus() == (status) (OK vs. INTERNAL: XLS_RET_CHECK failure (xls/dslx/deduce.cc:2192) channel_type != nullptr ((nil) vs. nullptr)`.

Fix plz