Fix incorrect migration notes for extensions without any migrations

When running migrations for an extension without any migrations (eg.
BBCode), the migration notes for the previous extension were being
displayed, because the Migrator never had a chance to clear them.

src/Extension/ExtensionManager.php

@@ -219,6 +219,7 @@ class ExtensionManager
      *
      * @param Extension $extension
      * @param bool|true $up
+     * @return array Notes from the migrator.
      */
     public function migrate(Extension $extension, $up = true)
     {
@@ -234,7 +235,11 @@ class ExtensionManager
             } else {
                 $this->migrator->reset($migrationDir, $extension);
             }
+
+            return $this->migrator->getNotes();
         }
+
+        return [];
     }
 
     /**