Resolve cod review feedback

Author: cormacpayne

src/Common/Commands.Common/MetricHelper.cs

@@ -68,7 +68,7 @@ private static string HashMacAddress
                                                nic.GetPhysicalAddress() != null &&
                                                !string.IsNullOrWhiteSpace(nic.GetPhysicalAddress().ToString()))?
                         .GetPhysicalAddress()?.ToString();
-                    _hashMacAddress = string.IsNullOrWhiteSpace(macAddress) ? null : GenerateSha256HashString(macAddress).Replace("-", string.Empty).ToLowerInvariant();
+                    _hashMacAddress = string.IsNullOrWhiteSpace(macAddress) ? null : GenerateSha256HashString(macAddress)?.Replace("-", string.Empty)?.ToLowerInvariant();
                 }
 
                 return _hashMacAddress;
@@ -249,7 +249,7 @@ public static string GenerateSha256HashString(string originInput)
         {
             if (string.IsNullOrWhiteSpace(originInput))
             {
-                return null;
+                return string.Empty;
             }
 
             using (var sha256 = new SHA256CryptoServiceProvider())

src/ResourceManager/Profile/Commands.Profile.Test/TelemetryTests.cs

@@ -25,11 +25,11 @@ public class TelemetryTests
     {
         [Fact]
         [Trait(Category.AcceptanceType, Category.CheckIn)]
-        public void HashOfNullOrWhitespaceValueReturnsNull()
+        public void HashOfNullOrWhitespaceValueReturnsEmptyString()
         {
-            Assert.Null(MetricHelper.GenerateSha256HashString(null));
-            Assert.Null(MetricHelper.GenerateSha256HashString(string.Empty));
-            Assert.Null(MetricHelper.GenerateSha256HashString(" "));
+            Assert.Equal(string.Empty, MetricHelper.GenerateSha256HashString(null));
+            Assert.Equal(string.Empty, MetricHelper.GenerateSha256HashString(string.Empty));
+            Assert.Equal(string.Empty, MetricHelper.GenerateSha256HashString(" "));
         }
 
         [Fact]
@@ -45,22 +45,12 @@ public void HashOfValidValueSucceeds()
 
         [Fact]
         [Trait(Category.AcceptanceType, Category.CheckIn)]
-        public void NetworkInterfaceWithEmptyAddressReturnsNull()
+        public void NetworkInterfaceWithEmptyAddressReturnsEmptyString()
         {
-            var networkInterfaceMock = new Mock<NetworkInterface>();
-            networkInterfaceMock.Setup(t => t.GetPhysicalAddress())
-                .Returns(
-                    () =>
-                        {
-                            return new PhysicalAddress(new byte[] { });
-                        }
-                );
-
-            var address = networkInterfaceMock.Object.GetPhysicalAddress();
-            Assert.NotNull(address);
+            var address = new PhysicalAddress(new byte[] { } );
             Assert.Equal(string.Empty, address.ToString());
             var hashAddress = MetricHelper.GenerateSha256HashString(address.ToString());
-            Assert.Null(hashAddress);
+            Assert.Equal(string.Empty, hashAddress);
         }
     }
 }