json: fix [raw] for option string (#17899)

vlib/json/json_raw_test.v

@@ -0,0 +1,38 @@
+import json
+
+struct TestOptionalRawString {
+	id   int
+	data ?string [raw]
+}
+
+fn test_raw_opt() {
+	test := TestOptionalRawString{
+		id: 1
+		data: 't
+e
+s
+t'
+	}
+	encoded := json.encode(test)
+	assert json.decode(TestOptionalRawString, encoded)!.data? == r'"t\ne\ns\nt"'
+}
+
+fn test_raw_none() {
+	test := TestOptionalRawString{
+		id: 1
+		data: none
+	}
+	encoded := json.encode(test)
+	r := json.decode(TestOptionalRawString, encoded)!.data
+	assert r == none
+}
+
+fn test_raw_empty_string() {
+	test := TestOptionalRawString{
+		id: 1
+		data: ''
+	}
+	encoded := json.encode(test)
+	r := json.decode(TestOptionalRawString, encoded)!.data or { 'z' }
+	assert r == '""'
+}