This should finally fix kv.test and dotty.test on the LIP6 buildfarm.

* src/tgbatest/kv.test, iface/nips/nipstest/dotty.test: Don't rely
on the ${DOT-...} syntax, because DOT is always set and might be
set to the empty value.

src/tgbatest/kv.test

@@ -27,7 +27,7 @@ set -e
 check ()
 {
   run 0 ../ltl2tgba -f -KV "$1" > out.dot
-  "${DOT-true}" out.dot > /dev/null
+  test -z "$DOT" || "$DOT" out.dot > /dev/null
   rm -f out.dot
 }