Add nullability guards to remaining comparison overlay tasks

The Blocking, Memory, and IO branches in the comparison overlay
section of CorrelatedTimelineLanesControl were missing the
`&& Result != null` guard the CPU and Wait branches already had.
Apply uniformly so a null result from `_dataService.Get*TrendAsync`
can't NRE on the subsequent .Select / .GroupBy.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erikdarlingdata

Lite/Controls/CorrelatedTimelineLanesControl.xaml.cs

@@ -182,19 +182,19 @@ await Task.WhenAll(cpuTask, waitTask, blockingTask, deadlockTask, memoryTask, fi
                     AddGhostLine(WaitStatsChart, refWaitTask.Result
                         .Select(d => (d.CollectionTime.AddMinutes(utcOffset).Add(timeShift).ToOADate(), d.WaitTimeMsPerSecond)).ToList(), "#FFB74D");
 
-                if (refBlockingTask.IsCompletedSuccessfully)
+                if (refBlockingTask.IsCompletedSuccessfully && refBlockingTask.Result != null)
                 {
                     var refBlocking = refBlockingTask.Result
                         .Select(d => (d.Time.AddMinutes(utcOffset).Add(timeShift).ToOADate(), (double)d.Count)).ToList();
                     if (refBlocking.Count > 0)
                         AddGhostLine(BlockingChart, refBlocking, "#E57373");
                 }
 
-                if (refMemoryTask.IsCompletedSuccessfully)
+                if (refMemoryTask.IsCompletedSuccessfully && refMemoryTask.Result != null)
                     AddGhostLine(MemoryChart, refMemoryTask.Result
                         .Select(d => (d.CollectionTime.AddMinutes(utcOffset).Add(timeShift).ToOADate(), d.BufferPoolMb)).ToList(), "#CE93D8");
 
-                if (refIoTask.IsCompletedSuccessfully)
+                if (refIoTask.IsCompletedSuccessfully && refIoTask.Result != null)
                 {
                     var refIo = refIoTask.Result
                         .GroupBy(d => d.CollectionTime)