add empty shape to custom keys

UI/GamepadEmu.h

@@ -284,6 +284,7 @@ namespace CustomKeyData {
 		{ ImageID("I_DIR"), ImageID("I_DIR_LINE"), 180.0f, false, false },
 		{ ImageID("I_DIR"), ImageID("I_DIR_LINE"), 0.0f, false, false },
 		{ ImageID("I_SQUARE_SHAPE"), ImageID("I_SQUARE_SHAPE_LINE"), 0.0f, false, false },
+		{ ImageID("I_EMPTY"), ImageID("I_EMPTY"), 0.0f, false, false },
 	};
 
 	// Button list

ui_atlasscript.txt

@@ -80,3 +80,4 @@ image I_SPEAKER source_assets/image/speaker.png copy
 image I_SPEAKER_MAX source_assets/image/speaker_max.png copy
 image I_SPEAKER_OFF source_assets/image/speaker_off.png copy
 image I_WINNER_CUP source_assets/image/winner_cup.png copy
+image I_EMPTY source_assets/image/empty.png copy