Fix psalm error (#1027)

ERROR: PossiblyNullArrayOffset - ../src/Curl/CaseInsensitiveArray.php:183:16 - Cannot access value on variable $this->keys using possibly null offset array-key|null (see https://psalm.dev/125)
        return $this->keys[$key] ?? $key;

src/Curl/CaseInsensitiveArray.php

@@ -180,7 +180,11 @@ class CaseInsensitiveArray implements \ArrayAccess, \Countable, \Iterator
     public function key()
     {
         $key = key($this->data);
-        return $this->keys[$key] ?? $key;
+        if ($key === null) {
+            return null;
+        } else {
+            return $this->keys[$key] ?? $key;
+        }
     }
 
     /**