Fix cell path when getting columns of non-records (#7508)

A follow-up to #7497. That change made it so that `get foo` would
eliminate non-record rows; I think that was an unintentional and
undesirable side-effect.

Before #7497:
```bash
〉[$nothing, { item: "foo" }] | get item
╭───┬─────╮
│ 0 │     │
│ 1 │ foo │
╰───┴─────╯
```
After #7497:
```bash
〉[$nothing, {item: "foo"}] | get item
╭───┬─────╮
│ 0 │ foo │
╰───┴─────╯
```

After this PR:
```bash
〉[$nothing, { item: "foo" }] | get item
╭───┬─────╮
│ 0 │     │
│ 1 │ foo │
╰───┴─────╯
```

cc: @merelymyself

crates/nu-command/tests/commands/open.rs

@@ -179,7 +179,7 @@ fn parses_json() {
 fn parses_xml() {
     let actual = nu!(
         cwd: "tests/fixtures/formats",
-        "open jonathan.xml | get rss.children.channel.children | get 0.item.children | get 0.link.children.0.0"
+        "open jonathan.xml | get rss.children.channel.children | get 0.item.children | get 3.link.children.3.0"
     );
 
     assert_eq!(