MOBILE-2512 workshop: Avoid error when downloading profiles in workshop

src/addon/mod/workshop/providers/prefetch-handler.ts

@@ -363,7 +363,9 @@ export class AddonModWorkshopPrefetchHandler extends CoreCourseActivityPrefetchH
             });
         }).then(() => {
             // Prefetch user profiles.
-            return this.userProvider.prefetchProfiles(userIds, courseId, siteId);
+            return this.userProvider.prefetchProfiles(userIds, courseId, siteId).catch(() => {
+                // Ignore errors.
+            });
         });
     }
 }

src/core/user/providers/user.ts

@@ -383,6 +383,10 @@ export class CoreUserProvider {
             promises = [];
 
         userIds.forEach((userId) => {
+            if (userId === null) {
+                return;
+            }
+
             userId = Number(userId); // Make sure it's a number.
 
             // Prevent repeats and errors.