Imported server bans cannot be !unbanned

[heftig]

Describe the bug
Server bans created by import from a room cannot be removed via the !unban command.

To Reproduce
Steps to reproduce the behavior:

1. Manually modify the ACL of an unprotected room to deny a server.
2. Tell Mjolnir !import <room> to import the ban from the room
3. Tell Mjolnir !unban server <server> apply to remove the ban

Expected behavior
The ban is applied and then removed from protected rooms.

Actual behavior
When Mjolnir imports the ban, it creates a state event of type org.matrix.mjolnir.rule.server.
When Mjolnir unbans, it creates an empty state event of type m.room.rule.server. This fails to remove the imported ban.

Additional context
Mjolnir v1.2.1
Synapse v1.52.0