[tei-council] change breaks DTD generation

[Lou Burnard]

Maybe it's just me, but it seems that the change introducedwith commit 
r11676 on 6 march by rahtz in order to "allow data.language to have an 
empty value, as per XML spec" has a somewhat undesirable effect on DTD 
generation. Namely my local build fails when I try generating the 
exemplars quite rightly protesting that "()" is not valid as the second 
part of an ATTDEF declaration thusly

Entity: line 1: parser error : Content error in the external subset
  %att.global.attributes;
                         ^
Entity: line 7:
xml:lang ()  #IMPLIED
                 ^
make[1]: *** [test-extns] Error 1

This provokes several questions in my mind not least

a) what the devil to do about it?

b) is mr jenkins falling down on the job again -- no sign of this being 
a problem in the last good build ?