Fix 'gather' rules in the python parser generator (GH-22021)

Currently, empty sequences in gather rules make the conditional for
gather rules fail as empty sequences evaluate as "False". We need to
explicitly check for "None" (the failure condition) to avoid false
negatives.

Lib/test/test_peg_generator/test_pegen.py

@@ -74,7 +74,7 @@ class TestPegen(unittest.TestCase):
             "Rule('term', 'int', Rhs([Alt([NamedItem(None, NameLeaf('NUMBER'))])]))"
         )
 
-    def test_repeat_with_separator_rules(self) -> None:
+    def test_gather(self) -> None:
         grammar = """
         start: ','.thing+ NEWLINE
         thing: NUMBER
@@ -85,6 +85,20 @@ class TestPegen(unittest.TestCase):
             "Rule('start', None, Rhs([Alt([NamedItem(None, Gather(StringLeaf(\"','\"), NameLeaf('thing'"
         ))
         self.assertEqual(str(rules["thing"]), "thing: NUMBER")
+        parser_class = make_parser(grammar)
+        node = parse_string("42\n", parser_class)
+        assert node == [
+            [[TokenInfo(NUMBER, string="42", start=(1, 0), end=(1, 2), line="42\n")]],
+            TokenInfo(NEWLINE, string="\n", start=(1, 2), end=(1, 3), line="42\n"),
+        ]
+        node = parse_string("1, 2\n", parser_class)
+        assert node == [
+            [
+                [TokenInfo(NUMBER, string="1", start=(1, 0), end=(1, 1), line="1, 2\n")],
+                [TokenInfo(NUMBER, string="2", start=(1, 3), end=(1, 4), line="1, 2\n")],
+            ],
+            TokenInfo(NEWLINE, string="\n", start=(1, 4), end=(1, 5), line="1, 2\n"),
+        ]
 
     def test_expr_grammar(self) -> None:
         grammar = """

Tools/peg_generator/pegen/python_generator.py

@@ -217,6 +217,9 @@ class PythonParserGenerator(ParserGenerator, GrammarVisitor):
                     else:
                         self.print("and")
                     self.visit(item)
+                    if is_gather:
+                        self.print("is not None")
+
             self.print("):")
             with self.indent():
                 action = node.action