checker: fix json.encode_pretty for struct expressions

[ttytm]

Fixes:

error: expression cannot be passed as `voidptr`
   20 | 
   21 | if !os.is_file(path) {
   22 |     os.write_file(path, json.encode_pretty(Config{}))!

_{Huly®: V_0.6-21340}

[spytheman]

Excellent work.