[11.x] Add default value for get and getHidden on Context

[michaelnabil230]

This PR adds a default value for get and getHidden on Context

Before:

$isUser = Context::get('is_user'); //  If is not has in array return null forever
$isUser = Context::getHidden('is_user'); //  If is not has in array return null forever

After:

$isUser = Context::get('is_user', false); // If is not has in array return false
$isUser = Context::getHidden('is_user', false); // If is not has in array return false