Substitute invalid UTF-8 in JSON error renderer output

[iliaal]

json_encode() returns false when any string in the payload contains an invalid UTF-8 sequence, so a JSON error response for an exception whose message or title carries raw binary data (invalid-UTF-8 user input echoed into exception messages is the common path) was emitted as a zero-length body under Content-Type: application/json. Strict API clients then fail to parse the empty body and the actual error information is silently discarded.

This adds JSON_INVALID_UTF8_SUBSTITUTE so invalid byte sequences are replaced with the UTF-8 replacement character instead of failing the whole encode. The renderer output stays valid JSON in all cases.

Reproduction on 4.x before this change:

$renderer = new \Slim\Error\Renderers\JsonErrorRenderer();
$output = $renderer(new \RuntimeException("bad \xB1\x31 bytes"), true);
var_dump(strlen($output)); // int(0)