[PHP 8.4] Fixes for implicit nullability deprecation

Fixes all issues that emits a deprecation notice on PHP 8.4.

See:
 - [RFC](https://wiki.php.net/rfc/deprecate-implicitly-nullable-types)
 - [PHP 8.4: Implicitly nullable parameter declarations deprecated](https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated)

Author: Ayesh

src/CommitFetcher.php

@@ -9,7 +9,7 @@ class CommitFetcher {
     private ?string $apiKey;
     private CurlFetcher $curlFetcher;
 
-    public function __construct(string $apiKey = null) {
+    public function __construct(?string $apiKey = null) {
         $this->apiKey = $apiKey;
         $this->curlFetcher = new CurlFetcher();
     }

src/NewsFetcher.php

@@ -21,7 +21,7 @@ class NewsFetcher {
 
     private array $staticReplacements = [];
 
-    public function __construct(string $apiKey = null) {
+    public function __construct(?string $apiKey = null) {
         $this->apiKey = $apiKey;
         $this->curlFetcher = new CurlFetcher();
     }

src/TagListFetcher.php

@@ -13,7 +13,7 @@ class TagListFetcher {
     private ?string $apiKey;
     private CurlFetcher $curlFetcher;
 
-    public function __construct(string $apiKey = null) {
+    public function __construct(?string $apiKey = null) {
         $this->apiKey = $apiKey;
         $this->curlFetcher = new CurlFetcher();
     }