Handle case of null BytesAllocatedPerOperation in the json (#5074)

This property is nullable on the BDN side: GetBytesAllocatedPerOperation -> https://github.com/dotnet/BenchmarkDotNet/blob/master/src/BenchmarkDotNet/Engines/GcStats.cs#L41. The ResultsComparer was crashing when trying to deserialize the json results.

Author: BrzVlad

src/tools/ResultsComparer/DataTransferContracts.cs

@@ -88,7 +88,7 @@ public class Memory
         public int Gen1Collections { get; set; }
         public int Gen2Collections { get; set; }
         public long TotalOperations { get; set; }
-        public long BytesAllocatedPerOperation { get; set; }
+        public long? BytesAllocatedPerOperation { get; set; }
     }
 
     public class Measurement

src/tools/ResultsComparer/MultipleInputsComparer.cs

@@ -103,10 +103,10 @@ private static string GetRatio((string id, Benchmark baseResult, Benchmark diffR
 
         private static string GetAllocatedDiff(Benchmark diffResult, Benchmark baseResult)
         {
-            long baseline = baseResult.Memory.BytesAllocatedPerOperation;
+            long baseline = baseResult.Memory.BytesAllocatedPerOperation ?? 0;
             if (baseline == 0)
                 baseline = GetMetricValue(baseResult);
-            long diff = diffResult.Memory.BytesAllocatedPerOperation;
+            long diff = diffResult.Memory.BytesAllocatedPerOperation ?? 0;
             if (diff == 0)
                 diff = GetMetricValue(diffResult);