getExistingDirectRoomWithUser

Return the roomId of an existing DM with the other user, or null if such room does not exist. A room is a DM if:

• it is listed in the m.direct account data

• the current user has joined the room

• the other user is invited or has joined the room

• it has exactly 2 members Note:

• the returning room can be encrypted or not

• the power level of the users are not taken into account. Normally in a DM, the 2 members are admins of the room