[3.9] Fix 'gather' rules in the python parser generator (GH-22021) (GH-22080)

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.
(cherry picked from commit e55a0e9)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

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