Issue #3456016: Fix implicitly nullable type declarations in Varbase API

src/Exception/KeyGenerationException.php

@@ -20,7 +20,7 @@ class KeyGenerationException extends \RuntimeException {
    * @param \Exception $previous
    *   Previous exception.
    */
-  public function __construct($message = "", $code = 0, \Exception $previous = NULL) {
+  public function __construct($message = "", $code = 0, ?\Exception $previous = NULL) {
     if (empty($message)) {
       $message = openssl_error_string() ?: 'An internal error occurred';
     }

src/OAuthKey.php

@@ -60,7 +60,7 @@ class OAuthKey {
    * @return bool
    *   TRUE if the key component(s) exist, FALSE otherwise.
    */
-  public function exists($which = NULL) {
+  public function exists(?string $which = NULL) {
     if ($which) {
       $key = $this->configFactory
         ->get('simple_oauth.settings')