fix(tvix/eval): handle builtins.split matching the empty string HEAD r/8247 canon

This prevents the following statements from looping endlessly:

```
builtins.split "(.*)" ""
builtins.split "([abc]*)" "abc"
builtins.split "(.*)" "abc"
builtins.split ".*" ""
```

Cover these (and some more examples) in the test suite.

Co-Authored-By: Florian Klink <flokli@flokli.de>
Change-Id: Ibd339f971e0f4e3e5c229816e2be5a8e3836fec9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11743
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

0 files changed, 0 insertions, 0 deletions