[texdoc] error on missing display

[Karl Berry]

I accidentally ran "texdoc etex" on a remote server where $DISPLAY
wasn't set, and got the following error msgs:

    xprop:  unable to open display ''
    usage:  xprop [-options ...] [[format [dformat]] atom] ...

    where options include:
    ...
../texlive/karl/Master/texmf/scripts/texdoc/config.tlu:388: bad argument #1 to 'find' (string expected, got nil)

Doesn't really matter, but it's not the friendliest :).