[PATCH] pgarchives: Bugfix: allow message's parentid to be null

Hello,

Upon execution of load_messages.py, when creating a new thread, I get a crash:
(Sorry about the format, I get this out of strace)

Traceback (most recent call last):\n  File \"/srv/pgarchives/local/loader/load_message.py\", lin\
e 174, in <module>\n    ap.store(conn, listid, opt.overwrite, opt.overwrite)\n  File
\"/srv/pgarchives/local/loader/li\
b/storage.py\", line 232, in store\n    'rawtxt': bytearray(self.rawtxt),\npsycopg2.errors.NotNullViolation: null
valu\
e in column \"parentid\" violates not-null constraint\nDETAIL:  Failing row contains (2, 2, cmatte
<cmatte@[my_domain]\
.tld>, helloworld@[my_pglister_domain], , Test archive after redeployment 2, 2021-10-25 14:13:32+00, 1635171212.3\
49997.1068459.nullmailer@[my_domain], hello world\n\n\n\n, null, f, null, \\x46726f6d2068656c6c6f776f726c642d6f776\
e65722b617263686976654062..., '2':5A 'after':3A 'archive':2A 'hello':6 'redeployment':4A 'test...).\n\n

I think the way to fix this is simply to allow message.parentid to be null.

Cheers,
-- 
Célestin Matte