Add new Function allow Null

[<martin.me.roberts@bt.com>]

This is to enable a similar functionality to the nillable function in
xsd, however the user would not have to supply the xsi:nil="true"
attribute.
 
 
This would mean that attributes could be empty i.e. no whitespace or any
characters between "".
For elements either the empty <empty/> format or the <empty></empty>
format would be acceptable.
 

Martin Roberts 

BT plc, 
Group Research 
t: +44 1473 609785 
e: martin.me.roberts@bt.com