[C#] Optional Scalars (#6217)

* [C#] Optional Scalars

* Moved scalar optional check to avoid null enum conversion

tests/optional_scalars/OptionalByte.cs

@@ -0,0 +1,16 @@
+// <auto-generated>
+//  automatically generated by the FlatBuffers compiler, do not modify
+// </auto-generated>
+
+namespace optional_scalars
+{
+
+public enum OptionalByte : sbyte
+{
+  None = 0,
+  One = 1,
+  Two = 2,
+};
+
+
+}