Check all asset types if type == null.

According to OpenFL's documentation, `type` is
"The asset type to match, or null to match any type."

src/lime/utils/AssetLibrary.hx

@@ -336,6 +336,10 @@ class AssetLibrary
 
 		return switch (cast(type, AssetType))
 		{
+			case null:
+				cachedBytes.exists(id) || cachedText.exists(id) || cachedImages.exists(id)
+					|| cachedAudioBuffers.exists(id) || cachedFonts.exists(id);
+
 			case IMAGE:
 				cachedImages.exists(id);
 
@@ -345,7 +349,8 @@ class AssetLibrary
 			case FONT:
 				cachedFonts.exists(id);
 
-			default: cachedBytes.exists(id) || cachedText.exists(id);
+			default:
+				cachedBytes.exists(id) || cachedText.exists(id);
 		}
 		#end
 	}