Return singleton empty string in _PyUnicode_FromASCII

GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = None
created_at = <Date 2016-10-09.15:49:06.266>
labels = ['type-feature', '3.7']
title = 'Return singleton empty string in _PyUnicode_FromASCII'
updated_at = <Date 2016-11-18.17:12:44.909>
user = 'https://github.com/zhangyangyu'

bugs.python.org fields:

activity = <Date 2016-11-18.17:12:44.909>
actor = 'xiang.zhang'
assignee = 'serhiy.storchaka'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2016-10-09.15:49:06.266>
creator = 'xiang.zhang'
dependencies = []
files = ['45038']
hgrepos = []
issue_num = 28398
keywords = ['patch']
message_count = 10.0
messages = ['278364', '278389', '278398', '280141', '280188', '281129', '281140', '281142', '281147', '281148']
nosy_count = 3.0
nosy_names = ['vstinner', 'serhiy.storchaka', 'xiang.zhang']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue28398'
versions = ['Python 3.7']