fix(Identifiable): handle null id values

Author: jaredcnance

src/JsonApiDotNetCore/Models/Identifiable.cs

@@ -37,6 +37,9 @@ public string StringId
         /// </summary>
         protected virtual string GetStringId(object value)
         {
+            if(value == null)
+                return string.Empty;
+
             var type = typeof(T);
             var stringValue = value.ToString();
 

test/UnitTests/Models/IdentifiableTests.cs

@@ -21,6 +21,16 @@ public void Setting_StringId_To_Null_Sets_Id_As_Default()
             Assert.Equal(0, resource.Id);
         }
 
-        private class IntId : Identifiable { }
+        [Fact]
+        public void GetStringId_Returns_EmptyString_If_Object_Is_Null()
+        {
+            var resource = new IntId();
+            var stringId = resource.ExposedGetStringId(null);
+            Assert.Equal(string.Empty, stringId);
+        }
+
+        private class IntId : Identifiable { 
+            public string ExposedGetStringId(object value) => GetStringId(value);
+        }
     }
 }