Fix bad exit status 255 generated by uncaught TypeError

The error_get_last function returns NULL if there hasn't been an error
yet, And the exit status 255 is reserved by PHP and shall be used when fatal
error occurred.

    PHP Fatal error:  Uncaught TypeError: Return value of TheSeer\phpDox\ErrorHandler::getLastError() must be of the type array,
    null returned in /net/enterprise-data1/home/ad min/gasolwu/Code/phpdox/src/shared/ErrorHandler.php:147
    Stack trace:
    #0 /net/enterprise-data1/home/admin/gasolwu/Code/phpdox/src/shared/ErrorHandler.php(55): TheSeer\phpDox\ErrorHandler->getLastError()
    theseer#1 [internal function]: TheSeer\phpDox\ErrorHandler->handleShutdown()
    theseer#2 {main}
      thrown in /net/enterprise-data1/home/admin/gasolwu/Code/phpdox/src/shared/ErrorHandler.php on line 147

References:
- https://secure.php.net/exit
- https://secure.php.net/error_get_last

src/shared/ErrorHandler.php

@@ -140,7 +140,7 @@ private function renderException($exception): void {
     private function getLastError(): array {
         $error = \error_get_last();
 
-        if ($error && $error['type'] === \E_USER_NOTICE) {
+        if ($error === null || $error['type'] === \E_USER_NOTICE) {
             return [];
         }