Safeguard rating distribution graph against null InputManager

[smoogipoo]

I don't know how to reproduce this:

[runtime] 2026-04-15 05:43:26 [verbose]: 📺 OsuScreenStack#478(depth:4) exit from ScreenQueue#281
[runtime] 2026-04-15 05:43:26 [verbose]: 📺 OsuScreenStack#478(depth:4) resume to ScreenIntro#687
[runtime] 2026-04-15 05:43:26 [verbose]: 📺 BackgroundScreenStack#692(depth:1) exit from MatchmakingBackgroundScreen#393
[runtime] 2026-04-15 05:43:26 [verbose]: 📺 BackgroundScreenStack#692(depth:1) resume to BackgroundScreenDefault#210
[runtime] 2026-04-15 05:43:26 [verbose]: 📺 OsuScreenStack#478(depth:3) exit from ScreenIntro#687
[runtime] 2026-04-15 05:43:26 [verbose]: 📺 OsuScreenStack#478(depth:3) resume to MainMenu#505
[runtime] 2026-04-15 05:43:26 [verbose]: 🌅 Global background change queued
[runtime] 2026-04-15 05:43:26 [verbose]: ButtonSystem's state changed from EnteringMode to TopLevel
[runtime] 2026-04-15 05:43:26 [debug]: Focus changed from nothing to DialogOverlay.
[runtime] 2026-04-15 05:43:26 [error]: An unhandled error has occurred.
[runtime] 2026-04-15 05:43:26 [error]: System.NullReferenceException: Object reference not set to an instance of an object.
[runtime] 2026-04-15 05:43:26 [error]: at osu.Game.Screens.OnlinePlay.Matchmaking.Queue.RatingDistributionGraph.get_TooltipContent() in /home/smgi/Repos/osu/osu.Game/Screens/OnlinePlay/Matchmaking/Queue/RatingDistributionGraph.cs:line 434
[runtime] 2026-04-15 05:43:26 [error]: at osu.Framework.Graphics.Cursor.IHasCustomTooltip`1.osu.Framework.Graphics.Cursor.IHasCustomTooltip.get_TooltipContent()
[runtime] 2026-04-15 05:43:26 [error]: at osu.Framework.Graphics.Cursor.TooltipContainer.hasValidTooltip(ITooltipContentProvider target)
[runtime] 2026-04-15 05:43:26 [error]: at osu.Framework.Graphics.Cursor.TooltipContainer.Update()
[runtime] 2026-04-15 05:43:26 [error]: at osu.Framework.Graphics.Drawable.UpdateSubTree()

As unplausible as it may seem, the only thing that can be null here is GetContainingInputManager(). The point of this exercise is to simply remove it by relying on OnMouseMove events instead.