optimize sieve

Sieve/Services/SieveProcessor.cs

@@ -268,6 +268,10 @@ namespace Sieve.Services
                 }
                 outerExpression = Expression.And(outerExpression, innerExpression);
             }
+            if (outerExpression==null)
+            {
+                return null;
+            }
             return Expression.Lambda<Func<TEntity, bool>>(outerExpression, parameterExpression);
         }
         private IQueryable<TEntity> ApplyFiltering<TEntity>(