Add BooleanArray tests for null and slice behavior (#9013)

This PR adds unit tests for BooleanArray to improve coverage of:

- new_null  behavior
- Slicing behavior when null values are present

There are no functional or API changes.

Tests run:
- cargo test -p arrow-array

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: UtkarshSahay123

arrow-array/src/array/boolean_array.rs

@@ -829,4 +829,32 @@ mod tests {
         assert_eq!(values.values(), &[0b1000_0000]);
         assert!(nulls.is_none());
     }
+
+    #[test]
+    fn test_new_null_array() {
+        let arr = BooleanArray::new_null(5);
+
+        assert_eq!(arr.len(), 5);
+        assert_eq!(arr.null_count(), 5);
+        assert_eq!(arr.true_count(), 0);
+        assert_eq!(arr.false_count(), 0);
+
+        for i in 0..5 {
+            assert!(arr.is_null(i));
+            assert!(!arr.is_valid(i));
+        }
+    }
+
+    #[test]
+    fn test_slice_with_nulls() {
+        let arr = BooleanArray::from(vec![Some(true), None, Some(false)]);
+        let sliced = arr.slice(1, 2);
+
+        assert_eq!(sliced.len(), 2);
+        assert_eq!(sliced.null_count(), 1);
+
+        assert!(sliced.is_null(0));
+        assert!(sliced.is_valid(1));
+        assert!(!sliced.value(1));
+    }
 }