fix warnings (#515)

Author: fretje

src/Host/Controllers/Identity/UsersController.cs

@@ -41,6 +41,7 @@ public Task<string> AssignRolesAsync(string id, UserRolesRequest request, Cancel
     {
         return _userService.AssignRolesAsync(id, request, cancellationToken);
     }
+
     [HttpPost]
     [TenantIdHeader]
     [MustHavePermission(FSHAction.Create, FSHResource.Users)]

src/Infrastructure/Multitenancy/Startup.cs

@@ -40,11 +40,11 @@ private static FinbuckleMultiTenantBuilder<FSHTenantInfo> WithQueryStringStrateg
         {
             if (context is not HttpContext httpContext)
             {
-                return Task.FromResult((string?)null!);
+                return Task.FromResult((string?)null);
             }
 
             httpContext.Request.Query.TryGetValue(queryStringKey, out StringValues tenantIdParam);
 
-            return Task.FromResult(tenantIdParam.ToString());
+            return Task.FromResult((string?)tenantIdParam.ToString());
         });
 }