Fix up signatures

Also add phpunit dependency, add test target, and fix up phpunit.xml.dist.

composer.json

@@ -10,7 +10,8 @@
     },
     "require-dev": {
         "roave/security-advisories": "dev-latest",
-        "yoast/phpunit-polyfills": "^1.0"
+        "yoast/phpunit-polyfills": "^1.0",
+        "phpunit/phpunit": "^9.6"
     },
     "archive" : {
         "exclude" : [
@@ -19,5 +20,8 @@
     },
     "autoload" : {
          "classmap": [ "src/"]
+    },
+    "scripts" : {
+        "test":  "phpunit"
     }
 }

phpunit.xml.dist

@@ -9,20 +9,10 @@
         stopOnFailure="false"
         processIsolation="false"
         backupGlobals="false"
-        syntaxCheck="true"
 >
     <testsuites>
         <testsuite name="Tests">
             <directory>./tests</directory>
         </testsuite>
     </testsuites>
-    <filter>
-        <whitelist addUncoveredFilesFromWhitelist="true">
-            <directory suffix=".php">./src</directory>
-            <exclude>
-                <directory>./vendor</directory>
-                <directory>./tests</directory>
-            </exclude>
-        </whitelist>
-    </filter>
 </phpunit>

src/OpenIDConnectClient.php

@@ -1223,9 +1223,9 @@ protected function urlEncode(string $str): string
     /**
      * @param string $jwt encoded JWT
      * @param int $section the section we would like to decode
-     * @return object
+     * @return ?object
      */
-    protected function decodeJWT(string $jwt, int $section = 0): stdClass {
+    protected function decodeJWT(string $jwt, int $section = 0): ?stdClass {
 
         $parts = explode('.', $jwt);
         return json_decode(base64url_decode($parts[$section]), false);
@@ -1727,7 +1727,7 @@ public function getAccessToken(): string
         return $this->accessToken;
     }
 
-    public function getRefreshToken(): string
+    public function getRefreshToken(): ?string
     {
         return $this->refreshToken;
     }