Fix null ref in aspnet TryGetTraceHeader

[SimonCropp]

was making it a pain to debug things when i was looking into the client os change

instead of null ref ex in PolyfillExtensions, we can early return

System.NullReferenceException: Object reference not set to an instance of an object.
   at PolyfillExtensions.Split(String str, Char c, StringSplitOptions options) in D:\Code\sentry-dotnet\src\Sentry\Internal\Polyfills.cs:line 23
   at Sentry.SentryTraceHeader.Parse(String value) in D:\Code\sentry-dotnet\src\Sentry\SentryTraceHeader.cs:line 47
   at Sentry.AspNet.HttpContextExtensions.TryGetTraceHeader(NameValueCollection headers) in D:\Code\sentry-dotnet\src\Sentry.AspNet\HttpContextExtensions.cs:line 20