Use MISSING sentinal rather than None for no files

Fixes #1884
Fixes BOT-1NY

Discord.py 2.0 changed how this works, webhooks now look for the MISSING sentinal, rather than None to determine whether files are being passed.
This was updated in this commit: Rapptz/discord.py@a6f7213

Author: ChrisLovering

bot/exts/moderation/incidents.py

@@ -105,7 +105,7 @@ async def make_embed(incident: discord.Message, outcome: Signal, actioned_by: di
         else:
             embed.set_author(name="[Failed to relay attachment]", url=attachment.proxy_url)  # Embed links the file
     else:
-        file = None
+        file = discord.utils.MISSING
 
     return embed, file