fix(iohandler): don't drop empty list/dict/tuple args to keep.* functions

keep/iohandler/iohandler.py

@@ -331,15 +331,11 @@ def _parse(self, tree):
                                 pass
                     else:
                         _arg = arg.id
-                    # if the value is empty '', we still need to pass it to the function
-                    # also, if the value is 0 or 0.0, we need to pass it to the function
-                    # 0 == False, so we need to check if the value is not False explicitly
-                    if (
-                        _arg
-                        or _arg == ""
-                        or (_arg == 0 or _arg == 0.0)
-                        and _arg is not False
-                    ):
+                    # _arg only ever stays None when none of the branches above
+                    # produced a value (e.g. a nested keep.* call returned nothing).
+                    # Any other parsed value - including "", 0, 0.0, False, [], {},
+                    # and () - is a legitimate argument and must be passed through.
+                    if _arg is not None:
                         _args.append(_arg)
 
                 # Parse keyword args

tests/test_iohandler.py

@@ -42,6 +42,27 @@ def test_with_function(context_manager):
     s = iohandler.render("hello keep.len({{ steps.some_list }})")
     assert s == "hello 3"
 
+
+@pytest.mark.parametrize(
+    "test_input, expected_output",
+    [
+        ("res keep.join([], '-')", "res "),
+        ("res keep.join({}, '-')", "res "),
+        ("res keep.len([])", "res 0"),
+    ],
+)
+def test_with_function_empty_container_argument(
+    context_manager, test_input, expected_output
+):
+    """
+    A literal [] or {} argument to a keep.* function must be passed through
+    as an empty list/dict, not silently dropped - see issue #6728.
+    """
+    iohandler = IOHandler(context_manager)
+    s = iohandler.render(test_input)
+    assert s == expected_output
+
+
 @pytest.mark.parametrize(
     "test_input, expected_output",
     [