json: fix option sumtype with int types (#18013)

vlib/json/json_encode_sumtype_test.v

@@ -0,0 +1,13 @@
+import json
+
+struct Test {
+	optional_sumtype ?MySumtype
+}
+
+type MySumtype = int | string
+
+fn test_simple() {
+	test := Test{}
+	encoded := json.encode(test)
+	assert dump(encoded) == '{}'
+}

vlib/v/gen/c/json.v

@@ -501,7 +501,11 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 					last_number_type = var_t
 					dec.writeln('\t\tif (cJSON_IsNumber(root)) {')
 					dec.writeln('\t\t\t${var_t} value = ${js_dec_name(var_t)}(root);')
-					dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value);')
+					if utyp.has_flag(.option) {
+						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}(&value) }, &${prefix}res, sizeof(${sym.cname}));')
+					} else {
+						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value);')
+					}
 					dec.writeln('\t\t}')
 				}
 			}