fix: allow transformDocument to return a partial document

Author: rexxars

src/types.ts

@@ -162,7 +162,7 @@ export type ExportOptions = {
    * @param doc - The document to transform
    * @returns The transformed document
    */
-  transformDocument?: (doc: SanityDocument) => SanityDocument
+  transformDocument?: (doc: SanityDocument) => Partial<SanityDocument>
 
   /**
    * Optional progress callback that will be called periodically during the export.