Change comparison operators to allow nulls (#8617)

Prior to this PR, the less/greater than operators (`<`, `>`, `<=`, `>=`)
would throw an error if either side was null. After this PR, these
operators return null if either side (or both) is null.

### Examples
```bash 
1 < 3       # true
1 < null    # null
null < 3    # null
null < null # null
```

### Motivation

JT [asked the C#
folks](https://discord.com/channels/601130461678272522/615329862395101194/1086137515053957140)
and this is apparently the approach they would choose for comparison
operators if they could start from scratch.

This PR makes `where` more convenient to use on jagged/missing data. For
example, we can now filter on columns that may not be present in every
row:
```
> [{foo: 123} {}] | where foo? > 10
╭───┬─────╮
│ # │ foo │
├───┼─────┤
│ 0 │ 123 │
╰───┴─────╯
```

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

@@ -194,3 +194,10 @@ fn fail_on_non_iterator() {
 
     assert!(actual.err.contains("only_supports_this_input_type"));
 }
+
+// Test that filtering on columns that might be missing/null works
+#[test]
+fn where_gt_null() {
+    let actual = nu!("[{foo: 123} {}] | where foo? > 10 | to nuon");
+    assert_eq!(actual.out, "[[foo]; [123]]");
+}