C#8 Nullable ref types - returnUrl field is required

Moved from https://github.com/dotnet/AspNetCore.Docs/issues/17145 by @djhmateer

When scaffolding out the Identity code and I have C#8 nullable ref types checking set in the .csproj

```
enable
```
I need to change the methods which have a default value set to null eg

```
public async Task OnGetAsync(string returnUrl = null)
```

to be a nullable string ie `string?` otherwise, the UI will set ModelState error giving returnUrl field is required

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* ID: 804e24dd-7e30-8cf5-2c99-1ffacb4582f6
* Version Independent ID: 3e25547b-e2c9-6634-2c38-2a77207f17d4
* Content: [Scaffold Identity in ASP.NET Core projects](https://docs.microsoft.com/en-gb/aspnet/core/security/authentication/scaffold-identity?view=aspnetcore-3.1&tabs=netcore-cli#scaffold-identity-into-a-razor-project-with-authorization)
* Content Source: [aspnetcore/security/authentication/scaffold-identity.md](https://github.com/dotnet/AspNetCore.Docs/blob/master/aspnetcore/security/authentication/scaffold-identity.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-security**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**