Specify a default ban reason

Fixes https://github.com/matrix-org/mjolnir/issues/20

src/commands/UnbanBanCommand.ts

@@ -113,7 +113,7 @@ export async function execBanCommand(roomId: string, event: any, mjolnir: Mjolni
     const ruleContent = {
         entity: bits.entity,
         recommendation,
-        reason: bits.reason,
+        reason: bits.reason || '<no reason supplied>',
     };
     const stateKey = `rule:${bits.entity}`;