Guard Activity.Current in GXSpanContext() to avoid NullReferenceException (#1291)

The parameterless GXSpanContext ctor dereferenced Activity.Current.Context with
no null check. When there is no active span — e.g. reading the current span
outside any started Activity, or in a CLI/batch procedure run (GXProcedure.
MainImpl) where no ambient span exists — Activity.Current is null and the ctor
threw a NullReferenceException (seen via SdtSpanContext.get_gxTpr_Spanid ->
new GXSpanContext()).

Use Activity.Current?.Context ?? default so an empty (all-zero) ActivityContext
is produced instead of crashing; TraceId/SpanId then return zeros rather than
throwing. Latent since the OpenTelemetry Spans API was added (#975), unrelated
to the recent dependency bump.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: claudiamurialdo

dotnet/src/dotnetcore/Providers/OpenTelemetry/Diagnostics/GXOtel.Diagnostics/OtelSpan.cs

@@ -118,7 +118,10 @@ public class GXSpanContext
 		private ActivityContext _activityContext;
 		public GXSpanContext()
 		{
-			_activityContext = Activity.Current.Context;
+			// Activity.Current is null when there is no active span (e.g. reading the current
+			// span outside any started Activity, or in a CLI/batch run with no span). Guard it
+			// so building a context yields an empty (all-zero) ActivityContext instead of an NRE.
+			_activityContext = Activity.Current?.Context ?? default;
 		}
 
 		public GXSpanContext(ActivityContext activityContext)