Remove null return from Code::fallback()

As it never returns null.

Author: szepeviktor

src/Adapters/Slides/Detectors/Code.php

@@ -16,7 +16,7 @@ public function detect(): ?EmbedCode
             ?: $this->fallback();
     }
 
-    private function fallback(): ?EmbedCode
+    private function fallback(): EmbedCode
     {
         $uri = $this->extractor->getUri();