json: fix option alias support (#18801)

vlib/json/json_option_alias_test.v

@@ -0,0 +1,41 @@
+import json
+
+struct Test {
+	optional_alias  ?MyAlias  // primitive
+	optional_struct ?MyAlias2 // complex
+}
+
+struct Complex {
+	a int = 3
+}
+
+type MyAlias = int
+type MyAlias2 = Complex
+
+fn test_empty() {
+	test := Test{}
+	encoded := json.encode(test)
+	assert dump(encoded) == '{}'
+	assert json.decode(Test, '{}')! == test
+}
+
+fn test_value() {
+	test := Test{
+		optional_alias: 1
+	}
+	encoded := json.encode(test)
+	assert dump(encoded) == '{"optional_alias":1}'
+	assert json.decode(Test, '{"optional_alias":1}')! == test
+}
+
+fn test_value_2() {
+	test := Test{
+		optional_alias: 1
+		optional_struct: Complex{
+			a: 1
+		}
+	}
+	encoded := json.encode(test)
+	assert dump(encoded) == '{"optional_alias":1,"optional_struct":{"a":1}}'
+	assert json.decode(Test, '{"optional_alias":1,"optional_struct":{"a":1}}')! == test
+}