/aspnet/samples/blob/main/samples/aspnet/Identity/AspNet.Identity.MySQL/UserStore.cs bad ArgumentNullExceptions #380

I was hunting for an example IUserClaimStore and the first thing that showed up was this sample. There's a few simple copy+paste errors.

AddClaimAsync and RemoveClaimAsync have ArgumentNullException("user"); for if (claim == null). It should be something like ArgumentNullException("claim");

The rest of the file appears correct.

It's a zero priority thing, but thought I'd toss it out there.