Support None comparisons for null expressions (#1489)

* Support None comparisons for null expressions

* Fold None comparison coverage into relational expr test

python/datafusion/expr.py

diff --git a/python/datafusion/expr.py b/python/datafusion/expr.py
index 7cd74ec..3200465 100644
--- a/python/datafusion/expr.py
+++ b/python/datafusion/expr.py
@@ -483,6 +483,8 @@
 
         Accepts either an expression or any valid PyArrow scalar literal value.
         """
+        if rhs is None:
+            return self.is_null()
         if not isinstance(rhs, Expr):
             rhs = Expr.literal(rhs)
         return Expr(self.expr.__eq__(rhs.expr))
@@ -492,6 +494,8 @@
 
         Accepts either an expression or any valid PyArrow scalar literal value.
         """
+        if rhs is None:
+            return self.is_not_null()
         if not isinstance(rhs, Expr):
             rhs = Expr.literal(rhs)
         return Expr(self.expr.__ne__(rhs.expr))

python/tests/test_expr.py

diff --git a/python/tests/test_expr.py b/python/tests/test_expr.py
index 1cf824a..d046eb4 100644
--- a/python/tests/test_expr.py
+++ b/python/tests/test_expr.py
@@ -153,8 +153,8 @@
 
     batch = pa.RecordBatch.from_arrays(
         [
-            pa.array([1, 2, 3]),
-            pa.array(["alpha", "beta", "gamma"], type=pa.string_view()),
+            pa.array([1, 2, 3, None]),
+            pa.array(["alpha", "beta", "gamma", None], type=pa.string_view()),
         ],
         names=["a", "b"],
     )
@@ -171,6 +171,10 @@
     assert df.filter(col("b") != "beta").count() == 2
 
     assert df.filter(col("a") == "beta").count() == 0
+    assert df.filter(col("a") == None).count() == 1  # noqa: E711
+    assert df.filter(col("a") != None).count() == 3  # noqa: E711
+    assert df.filter(col("b") == None).count() == 1  # noqa: E711
+    assert df.filter(col("b") != None).count() == 3  # noqa: E711
 
 
 def test_expr_to_variant():