Make ChangeOrgMemberRole handle missing userId

Since ChangeOrgMemberRole is called directly from GraphQL, it's possible
it could be passed a userId that isn't in our database. Ensure that it
can't accidentally create an invalid OrgMember entry in that case.

backend/LexBoxApi/GraphQL/OrgMutations.cs

@@ -101,6 +101,8 @@ public async Task<IQueryable<Organization>> ChangeOrgMemberRole(
         NotFoundException.ThrowIfNull(org);
 
         permissionService.AssertCanEditOrg(org);
+        var user = await dbContext.Users.FindAsync(userId);
+        NotFoundException.ThrowIfNull(user);
         await UpdateOrgMemberRole(dbContext, org, role, userId);
         return dbContext.Orgs.Where(o => o.Id == orgId);
     }