Apply strictNullChecks to src/utils/*!exportUtils (#10455

* Apply `strictNullChecks` to `src/utils/exportUtils`

* strict fix

* fix strictNullChecks issues in some utils

* fix error message

* test coverage

* lint

* more strictNullChecks

* small optimisation for getUniqueRoomsWithIndividuals

* tidy

* test coverage

src/utils/MediaEventHelper.ts

@@ -102,7 +102,10 @@ export class MediaEventHelper implements IDestroyable {
             }
         }
 
-        return fetch(this.media.thumbnailHttp).then((r) => r.blob());
+        const thumbnailHttp = this.media.thumbnailHttp;
+        if (!thumbnailHttp) return Promise.resolve(null);
+
+        return fetch(thumbnailHttp).then((r) => r.blob());
     };
 
     public static isEligible(event: MatrixEvent): boolean {