Refactor: Gist card: Fix description type since it can be not set (anuraghazra#3089)

Author: qwerty541

src/fetchers/types.d.ts

@@ -1,7 +1,7 @@
 export type GistData = {
   name: string;
   nameWithOwner: string;
-  description: string;
+  description: string | null;
   language: string | null;
   starsCount: number;
   forksCount: number;