Add check for null db parameter value (#161)

Author: HoberMellow

EF6.PG/NpgsqlServices.cs

@@ -60,6 +60,11 @@ protected override void SetDbParameterValue(DbParameter parameter, TypeUsage par
         // Npgsql > 4.0 does strict type checks on integral values and fails with enums passed with numeric DbType.
         static void ConvertValueToNumericIfEnum(DbParameter parameter)
         {
+            if (parameter.Value == null)
+            {
+                return;
+            }
+
             var parameterValueObjectType = parameter.Value.GetType();
 
             if (!parameterValueObjectType.IsEnum)