json: fix -cstrict build + optional map (#18014)

vlib/json/json_encode_map_test.v

@@ -0,0 +1,22 @@
+import json
+
+struct Test {
+	optional_string       ?string
+	optional_array        ?[]string
+	optional_struct_array ?[]string
+	optional_map          ?map[string]string
+}
+
+fn test_main() {
+	test := Test{}
+	encoded := json.encode(test)
+	assert dump(encoded) == '{}'
+
+	test2 := Test{
+		optional_map: {
+			'foo': 'bar'
+		}
+	}
+	encoded2 := json.encode(test2)
+	assert dump(encoded2) == '{"optional_map":{"foo":"bar"}}'
+}