fix double slash issue if no pathPrefix specified

Author: HermanHilbrektas

src/Storage.php

@@ -129,11 +129,11 @@ public function save($file, $preserveFileName = false, $overwrite = false, $conf
                     Yii::$app->security->generateRandomString(),
                     $fileObj->getExtension()
                 ]);
-                $path = implode(DIRECTORY_SEPARATOR, [$pathPrefix, $dirIndex, $filename]);
+                $path = implode(DIRECTORY_SEPARATOR, array_filter([$pathPrefix, $dirIndex, $filename]));
             } while ($this->getFilesystem()->has($path));
         } else {
             $filename = $fileObj->getPathInfo('filename');
-            $path = implode(DIRECTORY_SEPARATOR, [$pathPrefix, $dirIndex, $filename]);
+            $path = implode(DIRECTORY_SEPARATOR, array_filter([$pathPrefix, $dirIndex, $filename]));
         }
 
         $this->beforeSave($fileObj->getPath(), $this->getFilesystem());