Multiplayer lobby sometimes shows "Start game" when there is only one player

[pserwylo]

Describe the bug
Multiplayer lobby says "Ready to start" with a "Start game" button sometimes when there is only a single player in the lobby.

To Reproduce

• Start a multiplayer game with two players.
• Wait until the game is finished.
• Once back in the lobby (showing a "Start game" button), then have one player leave.
• The other player will still see "Ready to start" and "Start game"

Expected behavior
They should be returned to the "Waiting for other players" screen.