er7 to xml escape empty field

[isc-afuentes]

@PYDuquesnoy found an issue when trying to generate an empty xml field.

Proposed fix:

• Use "\X00" value in ER7 to generate an empty field when converting to XML.
• I've added 2.5_ORM-EmptyField unittest case, it works when converting er7->xml however it does not work when converting xml->er7.

Any thoughts on how can make the xml and er7 in the unittest case equivalent?
Maybe removing "\X00" before comparing the er7?