[CST-13245] Fix isMemberOf method

Author: atarix83

src/app/core/eperson/group-data.service.ts

@@ -56,7 +56,7 @@ export class GroupDataService extends IdentifiableDataService<Group> implements
   public subgroupsEndpoint = 'subgroups';
 
   private createData: CreateData<Group>;
-  private searchData: SearchData<Group>;
+  private searchData: SearchDataImpl<Group>;
   private patchData: PatchData<Group>;
   private deleteData: DeleteData<Group>;
 
@@ -91,9 +91,9 @@ export class GroupDataService extends IdentifiableDataService<Group> implements
     const options = new FindListOptions();
     options.searchParams = [new RequestParam('groupName', groupName)];
 
-    return this.searchBy(searchHref, options).pipe(
+    return this.findByHref(this.searchData.getSearchByHref(searchHref, options)).pipe(
       getRemoteDataPayload(),
-      map((groups: PaginatedList<Group>) => groups.totalElements > 0),
+      map((group: Group) => isNotEmpty(group)),
       catchError(() => observableOf(false)),
     );
   }