fix: Validation raises TypeError when invalid JSON comes

[kenjis]

Needs #8161

Description
Fixes #8134

• IncomingRequest will throw HTTPException when the body is invalid as JSON
• add lang key HTTP.invalidJSON.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

Seems like we need special treatment when the body is null.

[kenjis]

How?

When is the body null?
https://github.com/codeigniter4/CodeIgniter4/blob/develop/system/HTTP/IncomingRequest.php#L153
Ah, when sending 0 or ''as body?

[kenjis]

I sent #8161

[kenjis]

Seems like we need special treatment when the body is null.

Done.