Fixed crash with DbString parameter list on Oracle

Dapper NET40/SqlMapper.cs

@@ -1789,7 +1789,6 @@ public static void PackListParameters(IDbCommand command, string namePrefix, obj
                         count++;
                         var listParam = command.CreateParameter();
                         listParam.ParameterName = namePrefix + count;
-                        listParam.Value = item ?? DBNull.Value;
                         if (isString)
                         {
                             listParam.Size = 4000;
@@ -1805,6 +1804,7 @@ public static void PackListParameters(IDbCommand command, string namePrefix, obj
                         }
                         else
                         {
+                            listParam.Value = item ?? DBNull.Value;
                             command.Parameters.Add(listParam);
                         }
                     }
@@ -3610,4 +3610,4 @@ public partial class FeatureSupport
 
 #endif
 
-}
\ No newline at end of file
+}