fix: update category field to allow null values in posts collection schema

src/content.config.ts

@@ -11,7 +11,7 @@ const postsCollection = defineCollection({
 		description: z.string().optional().default(""),
 		image: z.string().optional().default(""),
 		tags: z.array(z.string()).optional().default([]),
-		category: z.string().optional().default(""),
+		category: z.string().optional().nullable().default(""),
 		lang: z.string().optional().default(""),
 
 		/* For internal use */