Fix value returned in error path of client.show_future

fixes 6223c9d

Author: YoshiRulz

src/BizHawk.Client.Common/lua/CommonLibs/ClientLuaLibrary.cs

@@ -291,7 +291,7 @@ public string ShowFuture(LuaFunction luaf, long maxFrames, string name = null)
 			if (maxFrames < 1 || maxFrames > short.MaxValue)
 			{
 				Log($"Invalid number of future frames ({maxFrames}); number must be positive and less than 2^15.");
-				return "";
+				return EventsLuaLibrary.EMPTY_UUID_STR;
 			}
 
 			INamedLuaFunction nlf = CreateAndRegisterNamedFunction(luaf, NamedLuaFunction.EVENT_TYPE_FUTURE, prohibitedApis: ApiGroup.PROHIBITED_MID_FRAME, name: name);

src/BizHawk.Client.Common/lua/LuaHelperLibs/EventsLuaLibrary.cs

@@ -10,7 +10,7 @@ namespace BizHawk.Client.Common
 	[Description("A library for registering lua functions to emulator events.\n All events support multiple registered methods.\nAll registered event methods can be named and return a Guid when registered")]
 	public sealed class EventsLuaLibrary : LuaLibraryBase, IRegisterFunctions
 	{
-		private static readonly string EMPTY_UUID_STR = Guid.Empty.ToString("D");
+		internal static readonly string EMPTY_UUID_STR = Guid.Empty.ToString("D");
 
 		public NLFAddCallback CreateAndRegisterNamedFunction { get; set; }