Fix City constructor from empty subdivisions

Fixes error:
```Cannot assign null to property GeoIp2\Model\City::$mostSpecificSubdivision of type GeoIp2\Record\Subdivision```

Which happens for example for IP from Hongkong.

Author: JarJak

src/Model/City.php

@@ -72,7 +72,7 @@ public function __construct(array $raw, array $locales = ['en'])
         $this->location = new Location($raw['location'] ?? []);
         $this->postal = new Postal($raw['postal'] ?? []);
 
-        if (!isset($raw['subdivisions'])) {
+        if (empty($raw['subdivisions'])) {
             $this->subdivisions = [];
             $this->mostSpecificSubdivision
                     = new Subdivision([], $locales);