Fixes Deprecated Notice in PHP 81

[socrates77]

Hi i pushed this fixes for the following notices when migrating to PHP 81

PHP Deprecated:  Return type of NL4WP_Container::offsetExists($offset) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Newsletter-for-Wordpress/includes/class-container.php on line 73
PHP Deprecated:  Return type of NL4WP_Container::offsetGet($offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Newsletter-for-Wordpress/includes/class-container.php on line 89
PHP Deprecated:  Return type of NL4WP_Container::offsetSet($offset, $value) should either be compatible with ArrayAccess::offsetSet(mixed $offset, mixed $value): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Newsletter-for-Wordpress/includes/class-container.php on line 108
PHP Deprecated:  Return type of NL4WP_Container::offsetUnset($offset) should either be compatible with ArrayAccess::offsetUnset(mixed $offset): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Newsletter-for-Wordpress/includes/class-container.php on line 124

[socrates77]

Added fix for the following deprecated error

PHP Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated in Newsletter-for-Wordpress/includes/api/xmlrpc.inc on line 2444

[socrates77]

Try to resolve this issue https://github.com/mailrouter/Newsletter-for-Wordpress/issues/7

[bago]

this should have been fixed in 4.5.11, thank you!