Fix for AppTemplate issue shopwareArchive#14

- add null as a possible return type in ShopRepositoryInterface
- adjust implementation of ShopRepository to return null if shop by id does not exist
- adjust RegistrationService to call create shop in db only if not exist yet

src/Shop/ShopRepository.php

@@ -33,7 +33,7 @@ public function createShop(ShopInterface $shop): void
         $queryBuilder->execute();
     }
 
-    public function getShopFromId(string $shopId): ShopInterface
+    public function getShopFromId(string $shopId): ?ShopInterface
     {
         $queryBuilder = $this->connection->createQueryBuilder();
         $queryBuilder->select('shop_id', 'shop_url', 'shop_secret', 'api_key', 'secret_key')
@@ -43,6 +43,10 @@ public function getShopFromId(string $shopId): ShopInterface
 
         $shop = $queryBuilder->execute()->fetchAssociative();
 
+        if ($shop === false) {
+            return null;
+        }
+
         return new ShopEntity(
             $shop['shop_id'],
             $shop['shop_url'],
@@ -80,4 +84,4 @@ public function deleteShop(ShopInterface $shop): void
 
         $queryBuilder->execute();
     }
-}
+}