Make expectedHash optional in prim_path

This fixes an error found in builtins.path that looks like:

store path mismatch in (possibly filtered) path added from '/private/tmp/nix-shell.CyXViH/nix-test/filter-source/filterin'

when no hash is specified

src/libexpr/primops.cc

@@ -1199,7 +1199,7 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
     string name;
     Value * filterFun = nullptr;
     auto method = FileIngestionMethod::Recursive;
-    Hash expectedHash(htSHA256);
+    std::optional<Hash> expectedHash;
 
     for (auto & attr : *args[0]->attrs) {
         const string & n(attr.name);