Test array access + avoid warning

src/Fields.php

@@ -117,21 +117,35 @@ public function getIterator(): Traversable
         return $iterator;
     }
 
+    /**
+     * @param string $offset
+     */
     public function offsetExists(mixed $offset): bool
     {
         return isset($this->fields[$offset]);
     }
 
+    /**
+     * @param string $offset
+     * @return true|Fields|null
+     */
     public function offsetGet(mixed $offset): mixed
     {
-        return $this->fields[$offset];
+        return $this->fields[$offset] ?? null;
     }
 
+    /**
+     * @param string $offset
+     * @param true|Fields $value
+     */
     public function offsetSet(mixed $offset, mixed $value): void
     {
         $this->fields[$offset] = $value;
     }
 
+    /**
+     * @param string $offset
+     */
     public function offsetUnset(mixed $offset): void
     {
         unset($this->fields[$offset]);

tests/FieldsTest.php

@@ -8,10 +8,26 @@
 use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
+use PHPUnit\Framework\TestStatus\Warning;
 
 #[CoversClass(Fields::class)]
 class FieldsTest extends TestCase
 {
+    public function testArrayAccess(): void
+    {
+        $fields = new Fields();
+
+        $this->assertNull($fields['@id']);
+
+        $fields['@id'] = true;
+
+        $this->assertTrue($fields['@id']);
+
+        $fields['eventDate'] = new Fields();
+
+        $this->assertInstanceOf(Fields::class, $fields['eventDate']);
+    }
+
     public function testKeys(): void
     {
         $fields = Fields::fromArray([