Copy presence bit for the delta case

Author: martinfantini

src/mfast/coder/encoder/encoder_field_operator.cpp

@@ -548,6 +548,7 @@ class delta_operator : public encoder_field_operator,
             cref.exponent() - bv.of_decimal.exponent_;
         delta_storage.of_decimal.mantissa_ =
             cref.mantissa() - bv.of_decimal.mantissa_;
+        delta_storage.of_decimal.present_ = cref.present();
 
         decimal_cref delta(&delta_storage, cref.instruction());
         stream << delta;