smtplib send_message should gives more clear exception if the msg is None

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-02.04:23:16.263>
labels = ['type-feature', 'library', '3.10']
title = 'smtplib send_message should gives more clear exception if the msg is None'
updated_at = <Date 2021-01-08.21:34:29.403>
user = 'https://github.com/oonid'

bugs.python.org fields:

activity = <Date 2021-01-08.21:34:29.403>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-01-02.04:23:16.263>
creator = 'oon'
dependencies = []
files = []
hgrepos = []
issue_num = 42807
keywords = []
message_count = 2.0
messages = ['384216', '384692']
nosy_count = 5.0
nosy_names = ['barry', 'terry.reedy', 'r.david.murray', 'maxking', 'oon']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42807'
versions = ['Python 3.10']