fix(tvix/eval): handle builtins.split matching the empty string

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>

tvix/eval/src/builtins/mod.rs

@@ -1286,6 +1286,9 @@ mod pure_builtins {
                 })
                 .collect();
             ret.push_back(Value::List(NixList::from(v)));
+            if pos == text.len() {
+                break;
+            }
             pos = thematch.end();
         }