Protect against claimsPrincipal being null (#7719)

## Summary of changes

We are getting `NullReferenceException` in `HttpContextSetUser`,
guessing the `claimsPrincipal` can be `null`.

## Reason for change

```
Error : Exception occurred when calling the CallTarget integration continuation.
System.NullReferenceException
   at Datadog.Trace.ClrProfiler.AutoInstrumentation.AspNetCore.UserEvents.HttpContextSetUser.OnMethodBegin[TTarget](TTarget instance, ClaimsPrincipal& claimsPrincipal)
   at Microsoft.AspNetCore.Http.DefaultHttpContext.set_User(ClaimsPrincipal value)
```

## Implementation details

Added `claimsPrincipal is not null`

## Test coverage

None 🤷 

## Other details
<!-- Fixes #{issue} -->


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---------

Co-authored-by: Andrew Lock <andrew.lock@datadoghq.com>

Author: bouwkast

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/AspNetCore/UserEvents/HttpContextSetUser.cs

@@ -43,13 +43,13 @@ public static class HttpContextSetUser
 
         private const string HttpContextExtensionsTypeName = "Microsoft.AspNetCore.Http.DefaultHttpContext";
 
-        internal static CallTargetState OnMethodBegin<TTarget>(TTarget instance, ref ClaimsPrincipal claimsPrincipal)
+        internal static CallTargetState OnMethodBegin<TTarget>(TTarget instance, ref ClaimsPrincipal? claimsPrincipal)
         {
             if (Security.Instance is { IsTrackUserEventsEnabled: true } security)
             {
                 var tracer = Tracer.Instance;
                 var scope = tracer.InternalActiveScope;
-                if (instance is HttpContext httpContext && scope is not null)
+                if (instance is HttpContext httpContext && scope is not null && claimsPrincipal is not null)
                 {
                     var span = scope.Span;
                     var foundUserId = false;