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();
         }
 

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-split.exp

@@ -0,0 +1 @@
+[ [ "" [ "a" ] "c" ] [ "" [ "a" ] "b" [ "c" ] "" ] [ "" [ "a" null ] "b" [ null "c" ] "" ] [ " " [ "FOO" ] " " ] [ "" [ "abc" ] "" [ "" ] "" ] [ "" [ "abc" ] "" [ "" ] "" ] [ "" [ ] "" ] ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-split.nix

@@ -0,0 +1,10 @@
+[
+  (builtins.split "(a)b" "abc")
+  (builtins.split "([ac])" "abc")
+  (builtins.split "(a)|(c)" "abc")
+  (builtins.split "([[:upper:]]+)" " FOO ")
+
+  (builtins.split "(.*)" "abc")
+  (builtins.split "([abc]*)" "abc")
+  (builtins.split ".*" "")
+]

tvix/glue/src/tests/tvix_tests/eval-okay-context-introspection.nix

@@ -71,7 +71,7 @@ in
   (builtins.hasAttr "allOutputs" (builtins.getContext drv.drvPath)."${builtins.unsafeDiscardStringContext drv.drvPath}")
   (legit-context == desired-context) # FIXME(raitobezarius): this should not use `builtins.seq`, this is a consequence of excessive laziness of Tvix, I believe.
   (reconstructed-path == combo-path)
-  # Those are too slow?
+  # These still fail with an internal error
   # (etaRule' "foo")
   # (etaRule' combo-path)
   (etaRule "foo")