Fix: ArgumentNullException in FormRequestPayloadExtractor when handling invalid form data on ASP.NET

[jamescrosswell]

Resolves #3701

Analysis

This is handled differently in ASP.NET vs ASP.NET Core.

In ASP.NET Core, the implementation for IHttpRequest is:

sentry-dotnet/src/Sentry.AspNetCore/HttpRequestAdapter.cs

Lines 16 to 18 in 2639611

	public IEnumerable<KeyValuePair<string, IEnumerable<string>>> Form =>
	_request?.Form.Select(k => new KeyValuePair<string, IEnumerable<string>>(k.Key, k.Value))
	?? Enumerable.Empty<KeyValuePair<string, IEnumerable<string>>>();

In ASP.NET the implementation is:

sentry-dotnet/src/Sentry.AspNet/Internal/SystemWebHttpRequest.cs

Lines 15 to 16 in e75d537

	public IEnumerable<KeyValuePair<string, IEnumerable<string>>>? Form
	=> _request.Form.AllKeys.Select(kv => new KeyValuePair<string, IEnumerable<string>>(kv, _request.Form.GetValues(kv)));

Seemingly it's only a problem in ASP.NET... pushing an initial PR so I can switch to my windows box for development.