[F] MA2Generator，遇到没有Note的音符时，会在计算Statistics时产生除0异常。

现在直接进行一次check，遇到0音符的谱面直接拒绝转换。

Author: Starrah

generator/mai/MA2Generator.cs

@@ -302,6 +302,11 @@ protected void GenerateStatistics(StringBuilder result, Statistics stats)
     {
         if (chart != null) throw new Exception(Locale.InstanceMultipleUsage);
         chart = _chart;
+        if (chart.Notes.Count == 0)
+        {
+            alerts.Add(new Alert(Error, Locale.NoNotesInChart));
+            throw new ConversionException(alerts);
+        }
         chart.Sort();
         StringBuilder result = new StringBuilder();