Fixed null check in external login.

(cherry picked from commit 9981907532)

src/Umbraco.Web.Common/Security/UmbracoSignInManager.cs

@@ -86,7 +86,7 @@ public abstract class UmbracoSignInManager<TUser> : SignInManager<TUser>
 
         var providerKey = auth.Principal.FindFirstValue(ClaimTypes.NameIdentifier);
         var provider = items[UmbracoSignInMgrLoginProviderKey];
-        if (providerKey == null || provider is not null)
+        if (providerKey == null || provider == null)
         {
             return null;
         }