Attribute value now accepts null string (#201)

***NO_CI***

Author: josesimoes

MetadataProcessor.Shared/Tables/nanoSignaturesTable.cs

@@ -817,7 +817,15 @@ private void WriteAttributeArgumentValue(
                         break;
                     case NanoCLRDataType.DATATYPE_STRING:
                         writer.Write((byte)nanoSerializationType.ELEMENT_TYPE_STRING);
-                        writer.Write(_context.StringTable.GetOrCreateStringId((string)argument.Value));
+                        if (argument.Value == null)
+                        {
+                            // write sentinel for null string
+                            writer.Write((ushort)0xFFFF);
+                        }
+                        else
+                        {
+                            writer.Write(_context.StringTable.GetOrCreateStringId((string)argument.Value));
+                        }
                         break;
                     default:
                         Debug.Fail(dataType.ToString());

MetadataProcessor.Tests/TestNFApp/MyClass1.cs

@@ -31,6 +31,11 @@ public void MyMethodWithData()
         {
         }
 
+        [DataRow(999, null)]
+        public void MyMethodWithDataRowWithNull()
+        {
+        }
+
         private readonly int _myField;
 
         [Ignore("I'm ignoring you!")]