> So, should programs be aware of "", or should getenv() return NULL if it
> finds "", or should such a situation simply be declared illegal?

Empty environment variables must be distinct from non-existant environment
variables, so I beleive that getenv should not return NULL if it finds "".

What an empty environment variable means, is of course dependant on
what program you are talking about.  In the case of EDITOR and vipw,
I see no real reason why "" should be special cased.  

EDITOR is supposed to contain the name of an editor to execute.  If it
does not, the exec will fail and the program should display an
appropriate error message.  It doesn't matter if the value of EDITOR
is "", "/dev/null", or any other bogus value.

	--jtc