fix for https://github.com/StackExchange/dapper-dot-net/issues/203

check to see if enumerable is null before access

Dapper NET40/SqlMapper.cs

@@ -2705,6 +2705,8 @@ public static void PackListParameters(IDbCommand command, string namePrefix, obj
                 var count = 0;
                 bool isString = value is IEnumerable<string>;
                 bool isDbString = value is IEnumerable<DbString>;
+                if (list != null)
+                {
                     foreach (var item in list)
                     {
                         count++;
@@ -2729,6 +2731,7 @@ public static void PackListParameters(IDbCommand command, string namePrefix, obj
                             command.Parameters.Add(listParam);
                         }
                     }
+                }
 
                 var regexIncludingUnknown = @"([?@:]" + Regex.Escape(namePrefix) + @")(?!\w)(\s+(?i)unknown(?-i))?";
                 if (count == 0)