add unit tests and fix exceptions (#3930)

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

         {
             var agentInfoProto = ai.ToAgentInfoProto();
 
-            var agentActionProto = new AgentActionProto
+            var agentActionProto = new AgentActionProto();
+            if(ai.storedVectorActions != null)
-                VectorActions = { ai.storedVectorActions }
-            };
+                agentActionProto.VectorActions.AddRange(ai.storedVectorActions);
+            }
 
             return new AgentInfoActionPairProto
             {
             var brainParametersProto = new BrainParametersProto
             {
                 VectorActionSize = { bp.VectorActionSize },
-                VectorActionSpaceType =
-                    (SpaceTypeProto)bp.VectorActionSpaceType,
+                VectorActionSpaceType = (SpaceTypeProto) bp.VectorActionSpaceType,
-            brainParametersProto.VectorActionDescriptions.AddRange(bp.VectorActionDescriptions);
+            if(bp.VectorActionDescriptions != null)
+            {
+                brainParametersProto.VectorActionDescriptions.AddRange(bp.VectorActionDescriptions);
+            }
             return brainParametersProto;
         }
 
         /// </summary>
         public static DemonstrationMetaProto ToProto(this DemonstrationMetaData dm)
         {
+            var demonstrationName = dm.demonstrationName ?? "";
             var demoProto = new DemonstrationMetaProto
             {
                 ApiVersion = DemonstrationMetaData.ApiVersion,
-                DemonstrationName = dm.demonstrationName
+                DemonstrationName = demonstrationName
             };
             return demoProto;
         }

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

     {
         public Action OnRequestDecision;
         ObservationWriter m_ObsWriter = new ObservationWriter();
-        public void RequestDecision(AgentInfo info, List<ISensor> sensors) {
-            foreach(var sensor in sensors){
+        public void RequestDecision(AgentInfo info, List<ISensor> sensors)
+        {
+            foreach (var sensor in sensors)
+            {
                 sensor.GetObservationProto(m_ObsWriter);
             }
             OnRequestDecision?.Invoke();
             agent1.SetPolicy(policy);
 
             StackingSensor sensor = null;
-            foreach(ISensor s in agent1.sensors){
-                if (s is  StackingSensor){
+            foreach (ISensor s in agent1.sensors)
+            {
+                if (s is  StackingSensor)
+                {
                     sensor = s as StackingSensor;
                 }
             }
             {
                 agent1.RequestDecision();
                 aca.EnvironmentStep();
-
             }
 
             policy.OnRequestDecision = () =>  SensorTestHelper.CompareObservation(sensor, new[] {18f, 19f, 21f});

com.unity.ml-agents/Tests/Editor/Communicator/GrpcExtensionsTests.cs

+using NUnit.Framework;
+using UnityEngine;
+using Unity.MLAgents.Policies;
+using Unity.MLAgents.Demonstrations;
+using Unity.MLAgents.Sensors;
+
+namespace Unity.MLAgents.Tests
+{
+    [TestFixture]
+    public class GrpcExtensionsTests
+    {
+        [Test]
+        public void TestDefaultBrainParametersToProto()
+        {
+            // Should be able to convert a default instance to proto.
+            var brain = new BrainParameters();
+            brain.ToProto("foo", false);
+        }
+
+        [Test]
+        public void TestDefaultAgentInfoToProto()
+        {
+            // Should be able to convert a default instance to proto.
+            var agentInfo = new AgentInfo();
+            agentInfo.ToInfoActionPairProto();
+            agentInfo.ToAgentInfoProto();
+        }
+
+        [Test]
+        public void TestDefaultDemonstrationMetaDataToProto()
+        {
+            // Should be able to convert a default instance to proto.
+            var demoMetaData = new DemonstrationMetaData();
+            demoMetaData.ToProto();
+        }
+    }
+}