fixed sieve filter

Sieve/Services/SieveProcessor.cs

@@ -269,7 +269,7 @@ namespace Sieve.Services
             }
             if (string.IsNullOrEmpty(fullQueryString))
             {
-                fullQueryString = " id > 0";
+                fullQueryString = " id != null";
             }
             return fullQueryString;
         }
@@ -379,8 +379,8 @@ namespace Sieve.Services
             if (outerExpression == null)
             {
                 var property = Expression.Property(parameterExpression, "id");
-                ConstantExpression constant = Expression.Constant(0, typeof(int));
-                var rst = Expression.GreaterThan(property, constant);
+                ConstantExpression constant = Expression.Constant(new Guid("{00000000-0000-0000-0000-000000000000}"), typeof(Guid));
+                var rst = Expression.NotEqual(property, constant);
                 return Expression.Lambda<Func<TEntity, bool>>(rst, parameterExpression);
             }
             return Expression.Lambda<Func<TEntity, bool>>(outerExpression, parameterExpression);