[Issue 24380] CTFE assert messages are replaced with "null" with -dip1008

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24380

--- Comment #1 from JR <zorael at gmail.com> ---
The behaviour changed in 2.101, before which the error message was
`"message"[0..7]`.

```
$ dub build --compiler=2.100/dmd 2>&1 | grep 'Error:'
source/app.d(17,9): Error: `"message"[0..7]`

$ dub build --compiler=2.101/dmd 2>&1 | grep 'Error:' 
source/app.d(17,9): Error: `null`
```

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