fix(tvix/eval): or should handle non-attrset values, too

If a nested attrpath encounters a non-set value, the sentinel value
denoting a lack of next values should be emitted. This mirrors the
behaviour of Nix.

Change-Id: Ia80443d5a11243cc6d98dcab1249a3f5fdf77e27
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6210
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.exp

@@ -0,0 +1 @@
+"works fine"

tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.nix

@@ -0,0 +1,2 @@
+# `or` operator should keep working if it encounters a non-set type.
+{ a.b = 42; }.a.b.c or "works fine"