Added isbn null check

src/main.ts

@@ -179,8 +179,8 @@ export default class BookTrackerPlugin extends Plugin {
 				genres: book.genres.map((g) => safeString(g)),
 				coverImageUrl: safeString(book.coverImageUrl),
 				pageCount: book.pageCount,
-				isbn: safeString(book.isbn),
-				isbn13: safeString(book.isbn13),
+				isbn: book.isbn === null ? null : safeString(book.isbn),
+				isbn13: book.isbn13 === null ? null : safeString(book.isbn13),
 			},
 		};