json: fix error for json decode with sumtype (#14025)

vlib/json/json_decode_with_sumtype_test.v

@@ -0,0 +1,13 @@
+import json
+
+type Test = []string | string
+
+struct Some {
+	t Test
+}
+
+fn test_json_decode_with_sumtype() ? {
+	v := json.decode(Some, '{"t": ["string", "string2"]}') ?
+	println(v)
+	assert '$v.t' == "Test(['string', 'string2'])"
+}