default value + mandatory attribute is a strange combination but it used in some known templates (i.e. euronext)

Here is a proposal:

class: org.openfast.template.Group
method: encode

FieldValue fieldValue = groupValue.getValue(fieldIndex);
Field field = getField(fieldIndex);

//*************************************************************************************************************************
if (field instanceof Scalar && fieldValue == null && ((Scalar) field).getOperator() == Operator.DEFAULT)
fieldValue = ((Scalar) field).getDefaultValue();
//*************************************************************************************************************************

if (!field.isOptional() && fieldValue == null)
Global.handleError(FastConstants.GENERAL_ERROR, "Mandatory field " + field + " is null");