The python PEG generator incorrectly handles empty sequences in gather rules

[pablogsal]

BPO	41667
Nosy	@lysnikolaou, @pablogsal
PRs	bpo-41667: Fix 'gather' rules in the python parser generator #22021

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-10-19.17:08:46.807>
created_at = <Date 2020-08-30.20:08:36.266>
labels = ['3.9', '3.10']
title = 'The python PEG generator incorrectly handles empty sequences in gather rules'
updated_at = <Date 2020-10-19.17:09:38.435>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2020-10-19.17:09:38.435>
actor = 'lys.nikolaou'
assignee = 'none'
closed = True
closed_date = <Date 2020-10-19.17:08:46.807>
closer = 'lys.nikolaou'
components = []
creation = <Date 2020-08-30.20:08:36.266>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 41667
keywords = ['patch']
message_count = 1.0
messages = ['376129']
nosy_count = 2.0
nosy_names = ['lys.nikolaou', 'pablogsal']
pr_nums = ['22021']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue41667'
versions = ['Python 3.9', 'Python 3.10']

[pablogsal]

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.