refactor: Update SlugFile type definition to include optional firstImage field

- Moved the firstImage property to the end of the SlugFile type definition for better organization.
- Updated the remarkExtractFirstImage function to return undefined when no image is found, improving clarity in the image extraction logic.

Author: HazAT

src/mdx.ts

@@ -45,13 +45,13 @@ import {isNotNil} from './utils';
 import {isVersioned, VERSION_INDICATOR} from './versioning';
 
 type SlugFile = {
-  firstImage?: string;
   frontMatter: Platform & {slug: string};
   matter: Omit<matter.GrayMatterFile<string>, 'data'> & {
     data: Platform;
   };
   mdxSource: string;
   toc: TocNode[];
+  firstImage?: string;
 };
 
 const root = process.cwd();

src/remark-extract-first-image.ts

@@ -25,6 +25,7 @@ export default function remarkExtractFirstImage(options: {exportRef: string[]})
         firstImage = node.url;
         return false; // Stop visiting once we find the first image
       }
+      return undefined;
     });
 
     // Export the first image URL if found