cgen: fix json encoding of structs with option fields (skip the fields with a value of none) (#16916)

vlib/json/json_encode_struct_with_option_field_test.v

@@ -1,4 +1,5 @@
 import json
+import x.json2
 
 struct Foo {
 	name string
@@ -11,10 +12,18 @@ fn test_json_encode_struct_with_option_field() {
 	}
 	ret1 := json.encode(f1)
 	println(ret1)
-	assert ret1 == '{"name":"hello","num":null}'
+	assert ret1 == '{"name":"hello"}'
+
+	ret2 := json2.encode(f1)
+	println(ret2)
+	assert ret2 == '{"name":"hello"}'
 
 	f2 := Foo{'hello', 22}
-	ret2 := json.encode(f2)
-	println(ret2)
-	assert ret2 == '{"name":"hello","num":22}'
+	ret3 := json.encode(f2)
+	println(ret3)
+	assert ret3 == '{"name":"hello","num":22}'
+
+	ret4 := json2.encode(f2)
+	println(ret4)
+	assert ret4 == '{"name":"hello","num":22}'
 }