fix Demonstration tests (#3554)

* fix Demonstration tests

* handle null compressed obs in GetObservationProto

* throw exception if sensor.GetCompressedObservation returns null

com.unity.ml-agents/Runtime/Communicator/GrpcExtensions.cs

             }
             else
             {
+                var compressedObs = sensor.GetCompressedObservation();
+                if (compressedObs == null)
+                {
+                    throw new UnityAgentsException(
+                        $"GetCompressedObservation() returned null data for sensor named {sensor.GetName()}. " +
+                        "You must return a byte[]. If you don't want to use compressed observations, " +
+                        "return SensorCompressionType.None from GetCompressionType()."
+                        );
+                }
+
-                    CompressedData = ByteString.CopyFrom(sensor.GetCompressedObservation()),
+                    CompressedData = ByteString.CopyFrom(compressedObs),
                     CompressionType = (CompressionTypeProto)sensor.GetCompressionType(),
                 };
             }

com.unity.ml-agents/Tests/Editor/MLAgentsEditModeTest.cs

         public byte[] GetCompressedObservation()
         {
             numCompressedCalls++;
-            return null;
+            return new byte[] { 0 };
         }
 
         public SensorCompressionType GetCompressionType()