THRIFT-4868: Fix Go compilation for optional sets with default values (#1802)

Client: go
diff --git a/test/ThriftTest.thrift b/test/ThriftTest.thrift
index d1e6b5e..99a15ff 100644
--- a/test/ThriftTest.thrift
+++ b/test/ThriftTest.thrift
@@ -409,3 +409,7 @@
   1: optional StructA aa;
   2: required StructA ab;
 }
+
+struct OptionalSetDefaultTest {
+  1: optional set<string> with_default = [ "test" ]
+}