[HocVoNgThai]

When exporting JSON with --natural-utf8, flatc writes some control characters directly into string values instead of escaping them. The command succeeds, but the generated JSON is invalid and cannot be read by standard JSON parsers.

For example, this valid input:

{"text": "café\u0000menu"}

is exported with a raw NUL byte in place of \u0000, causing JSON parsing to fail.

This fix preserves \u0000 as an escape sequence while keeping café readable, producing valid JSON without losing the original string content.

The regression test covers all 32 control characters, checks that Unicode remains readable, and verifies that the generated JSON can be parsed again.

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