fix: php 7.4 syntax error

Author: sooluh

src/PDDikti.php

@@ -11,10 +11,19 @@ class PDDikti
 
     private string $nim;
     private string $prodi;
-    private string|null $id = null;
-    private string|null $name = null;
-    private string|null $gender = null;
-    private bool|null $isGraduated = null;
+
+    /** @var string|null */
+    private ?string $id = null;
+
+    /** @var string|null */
+    private ?string $name = null;
+
+    /** @var string|null */
+    private ?string $gender = null;
+
+    /** @var bool|null */
+    private ?bool $isGraduated = null;
+
     // STT Wastukancana PT ID
     private string $pt = '2CE2EA61-3574-43CA-81D5-E8EF77B6DDF7';