Improve handling of missing output of virtualenv version fact command

Partially fixes #527

Signed-off-by: Wiebe Verweij <[email protected]>

Author: wiebe

lib/facter/virtualenv_version.rb

@@ -3,7 +3,8 @@
 Facter.add('virtualenv_version') do
   setcode do
     if Facter::Util::Resolution.which('virtualenv')
-      Facter::Util::Resolution.exec('virtualenv --version 2>&1').match(%r{(\d+\.\d+\.?\d*).*$})[1]
+      results = Facter::Util::Resolution.exec('virtualenv --version 2>&1').match(%r{(\d+\.\d+\.?\d*).*$})
+      results[1] if results
     end
   end
 end