cgen: fix ptr field encoding for json (fix #22717) (#22720)

vlib/json/json_encode_ptr_test.v

@@ -0,0 +1,31 @@
+import json
+
+struct Number {
+	min int
+	max int
+}
+
+pub struct Resp {
+pub:
+	options []string @[omitempty]
+	number  &Number = unsafe { nil }  @[omitempty]
+}
+
+fn (r Resp) str() string {
+	return json.encode(r)
+}
+
+fn test_main() {
+	r1 := Resp{
+		options: ['first', 'second']
+	}
+	r2 := Resp{
+		number: &Number{0, 0}
+	}
+	r3 := Resp{
+		number: &Number{1, 2}
+	}
+	assert r1.str() == '{"options":["first","second"]}'
+	assert r2.str() == '{"number":{"min":0,"max":0}}'
+	assert r3.str() == '{"number":{"min":1,"max":2}}'
+}