make guild count nullable

bot.py

@@ -64,7 +64,7 @@ class Bot(Bot):
 		user_id_md5 = hashlib.md5(ctx.author.id.to_bytes(8, byteorder='big'), usedforsecurity=False).digest()
 		await self.pool.execute(
 			'INSERT INTO invokes (guild_id, user_id_md5, command) VALUES ($1, $2, $3)',
-			ctx.guild.id, user_id_md5, ctx.command.qualified_name,
+			getattr(ctx.guild, 'id', None), user_id_md5, ctx.command.qualified_name,
 		)
 
 	# we use on_shard_ready rather than on_ready because the latter is a bit less reliable

schema.sql

@@ -2,7 +2,8 @@ SET TIME ZONE 'UTC';
 
 -- inserted for every time a command is invoked
 CREATE TABLE invokes (
-	guild_id BIGINT NOT NULL,
+	-- nullable because it may be invoked in DMs
+	guild_id BIGINT,
 	-- hashed for privacy
 	user_id_md5 BYTEA NOT NULL,
 	-- the qualified name of the command invoked (https://discordpy.readthedocs.io/en/stable/ext/commands/api.html?highlight=qualified_name#discord.ext.commands.Command.qualified_name)