fix: fix a regression of adding comment only line to an array

Fix #223

tests/test_items.py

@@ -475,6 +475,7 @@ def test_array_add_line():
     t.add_line(1, 2, 3, comment="Line 1")
     t.add_line(4, 5, 6, comment="Line 2")
     t.add_line(7, api.ws(","), api.ws(" "), 8, add_comma=False)
+    t.add_line(comment="Line 4")
     t.add_line(indent="")
     assert len(t) == 8
     assert list(t) == [1, 2, 3, 4, 5, 6, 7, 8]
@@ -484,6 +485,7 @@ def test_array_add_line():
     1, 2, 3, # Line 1
     4, 5, 6, # Line 2
     7, 8,
+    # Line 4
 ]"""
     )
 
@@ -864,14 +866,16 @@ def test_table_copy():
 
 
 def test_copy_copy():
-    result = parse("""
+    result = parse(
+        """
     [tool.poetry]
     classifiers = [
     # comment
         "a",
         "b",
     ]
-    """)
+    """
+    )
     classifiers = result["tool"]["poetry"]["classifiers"]
     new = copy.copy(classifiers)
     assert new == classifiers

tomlkit/items.py

@@ -1298,7 +1298,14 @@ def add_line(
         list.extend(self, data_values)
         if len(self._value) > 0:
             last_item = self._value[-1]
-            last_value_item = next((v for v in self._value[::-1] if v.value), None)
+            last_value_item = next(
+                (
+                    v
+                    for v in self._value[::-1]
+                    if v.value is not None and not isinstance(v.value, Null)
+                ),
+                None,
+            )
             if last_value_item is not None:
                 last_value_item.comma = Whitespace(",")
             if last_item.is_whitespace():