getAttributeValue() emits null-array-offset deprecation on PHP 8.4+ #19

InputDataContainer::getAttributeValue() emits a PHP deprecation on every call made without an explicit $key.

Where

src/InputDataContainer.php (line 37 in the released 1.2.3):

public function getAttributeValue($key = null)
{
    ...
    } elseif (isset($data[$key])) {   // $key is null on the default call

Why it fires

As of PHP 8.4/8.5, isset() is no longer exempt from the null-array-offset deprecation:

$ php -r '$d = ["a" => 1]; $k = null; var_dump(isset($d[$k]));'
PHP Deprecated:  Using null as an array offset is deprecated, use an empty string instead
bool(false)

Verified on PHP 8.5.9. Both isset($data[$key]) and the later $data[$this->attributeKey] read are affected when the value is null.

Impact

Every validated request in a consuming plugin logs the notice. In Bit CRM it also leaks into the HTTP response body — the REST envelope's additional field carries the raw deprecation text, e.g.:

{"status":"success","code":"SUCCESS","data":"Note created successfully!",
 "additional":"\nDeprecated: Using null as an array offset is deprecated, use an empty string instead in .../wp-validator/src/InputDataContainer.php on line 37"}

Suggested fix

Normalize the offset before use, e.g. $key ?? '', or guard with $key !== null && isset($data[$key]). The attributeKey read on the following branch needs the same treatment (it is already null-coalesced at line 31 for trim(), but not where it is used as an offset).

Reproduce

Call any validator rule path that reaches getAttributeValue() with no argument under PHP 8.4+.