Fix document resolver interface (#48)

Author: lukmzig

src/Lib/DocumentResolver.php

@@ -23,12 +23,12 @@
 /**
  * @internal
  */
-final class DocumentResolver
+final class DocumentResolver implements DocumentResolverInterface
 {
     /**
      * @throws Exception
      */
-    public function getInstance(string $adapter = null): Adapter
+    public function getInstance(string $adapter = null): ?Adapter
     {
         Document::getInstance($adapter);
     }

src/Lib/DocumentResolverInterface.php

@@ -24,7 +24,7 @@ interface DocumentResolverInterface
     /**
      * @throws Exception
      */
-    public function getInstance(string $adapter = null): Adapter;
+    public function getInstance(string $adapter = null): ?Adapter;
 
     public function isAvailable(): bool;