Crash with `getcompletiontype()`

### Steps to reproduce

1. `vim --clean`
2. `:echo getcompletiontype('ls ')`
3. Crash

### Expected behaviour

No crash

### Version of Vim

VIM - Vi IMproved 9.1  Included patches: 1-1512

### Environment

Manjaro Linux
Alacritty alacritty 0.15.1 (0c405d53)

### Logs and stack traces

```shell

```