Add null check when displaying project icons for MapContentView.

[m1cxzfw3q]

The game crashes because the new project icon is empty.

java.lang.NullPointerException: Cannot invoke "arc.graphics.g2d.TextureRegion.found()" because "u.uiIcon" is null
	at mindustry.editor.data.MapContentView.build(MapContentView.java:85)
	at mindustry.editor.data.MapAssetsDialog.rebuild(MapAssetsDialog.java:130)
	at mindustry.editor.data.MapContentView.lambda$addContent$28(MapContentView.java:188)
	at mindustry.core.UI.lambda$loadAnd$10(UI.java:295)
	at arc.util.Timer$1.run(Timer.java:88)
	at arc.util.TaskQueue.run(TaskQueue.java:17)
	at arc.backend.sdl.SdlApplication.loop(SdlApplication.java:216)
	at arc.backend.sdl.SdlApplication.<init>(SdlApplication.java:58)
	at mindustry.desktop.DesktopLauncher.main(DesktopLauncher.java:53)

So, this PR solves this issue by adding empty check for the image display part.

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[Anuken]

uiIcon should never be allowed to be null, and never appears as null in my tests. A null icon would crash other parts of the game. Can you provide steps to reproduce this in-game?

[m1cxzfw3q]

Create buildings with errors in the game by data patches.

Even with fullIcon and uiIcon configured, it will crash.

[Anuken]

The root cause has been fixed, but I added an extra check just in case.