Reinstate `GameInfo.NullInstance`'s name, and only instantiate it once

reverts 8dcea2470
`PathEntryCollection.RetroSystemAbsolutePath` extension didn't like this, there
may be others, and I don't want to look through them all

src/BizHawk.Emulation.Common/Database/GameInfo.cs

@@ -46,9 +46,9 @@ namespace BizHawk.Emulation.Common
 			return ret;
 		}
 
-		public static GameInfo NullInstance => new GameInfo
+		public static readonly GameInfo NullInstance = new()
 		{
-			Name = string.Empty,
+			Name = "Null",
 			System = VSystemID.Raw.NULL,
 			Hash = "",
 			Region = "",