ALTAPPS-1308: Shared speed up code submissions evaluation for beginners by removing linters

[ivan-magda]

YouTrack Issues:
#ALTAPPS-1308

Checklist

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Description

1. Adds "lint_profile": null in the reply for code problems.
2. Our JSON configuration does not encode defaults (code, docs), so I made Reply.lintProfile with a String? type and a default value of an empty string to be able to serialize null.