model: Make message field optional in Release model

A release may have an empty message, for instance those derived from a Mercurial repository.

12:45 $ psql service=swh-replica
psql (11.7 (Debian 11.7-0+deb10u1))
Connexion SSL (protocole : TLSv1.3, chiffrement : TLS_AES_256_GCM_SHA384, bits : 256, compression : désactivé)
Saisissez « help » pour l'aide.

softwareheritage=> select id from release where comment is NULL limit 10;
                     id                     
--------------------------------------------
 \xc581a993393d1b61fc7f3964014313734cb4fec1
 \xdab5a0841cf00e46861e15198dd0e57f1a5ca5f9
 \xd39288723941ecaf18b5231377c9885a446d6b59
 \xc61eaa2cbd1438ae60551b8ef3aea2c5da939aae
 \x7acc3693e4661e419d140769f18fb4d2084c7db2
 \xbed8489aedbe51e646440cfd5b49f2f43eb20555
 \x68f7ceeaea00deddbf15e17bb1414eb7725c89d7
 \xc471bbed0a05d8f822a820744f05754140c51893
 \x3d74278a26b139adcee267a49d1542c14dfe1f31
 \x39eeabe99031ba4803986e7dfc816ec15b4151c4
(10 lignes)

So make that field optional to avoid type validation errors.

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Migrated from D2945 (view on Phabricator)