gvpr -c option doesn't work anymore on empty program

[Swann Perarnau]

Binary package hint: graphviz

Expected Behavior:
The man page of gvpr contains a "nop" example: the command
gvpr -c '' input.dot
should print the input graph, unmodified.

This "nop" behavior allows someone writing dot files by hand to validate them
easily. I use it myself in automated tests of a dot conversion program.

Bug:
The last version of gvpr doesn't print anything on such command.

System Info:
~$ lsb_release -rd
Description: Ubuntu 10.10
Release: 10.10
~$ apt-cache policy graphviz
graphviz:
  Installed: 2.26.3-4
  Candidate: 2.26.3-4
  Version table:
 *** 2.26.3-4 0
        500 http://fr.archive.ubuntu.com/ubuntu/ maverick/main i386 Packages
        100 /var/lib/dpkg/status