Unexpected `Null` behavior for `ScalarValue::partial_cmp()`

### Describe the bug

For `partial_cmp()` function for `ScalarValue`

https://github.com/apache/datafusion/blob/79f67b8ef2d3016c866ae92d7d2f56d1a7830045/datafusion/common/src/scalar/mod.rs#L568

The current behavior is unexpected:

```rust
// this test can be added to the above file that `partial_cmp()` lives
#[test]
fn scalar_partial_ordering_nulls() {
use ScalarValue::*;

assert_eq!(
Int32(Some(3)).partial_cmp(&Int32(None)),
Some(Ordering::Greater)
);
assert_eq!(
Int32(None).partial_cmp(&Int32(Some(3))),
Some(Ordering::Less)
);
}
```
The comparison between `ScalarValue { type: Int32, val: 3 }` and `ScalarValue { type: Int32, val: Null }` returns `Some(_)`, whereas I expected `None`, indicating that the values are not comparable, consistent with SQL’s three-valued logic.

Additionally, comparing `Null` with `Null` returns `Some(Ordering::Equal)` instead of `None`.

We should either:
1. Fix this behavior to follow SQL’s null comparison semantics, or
2. If this non-standard null behavior is intentional for specific reasons, explicitly document it in the comments.

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

Start in datafusion/common/src/scalar/mod.rs at the partial_cmp implementation referenced in the issue, then review the surrounding tests and comments. Add the proposed scalar_partial_ordering_nulls test to observe the current behavior, and confirm whether the intended result is SQL-style incomparability or documented existing behavior before making the issue's outcome explicit.

Written by the indexing model from the issue text.