MINOR: sample: improve error reporting on missing arg to strcmp() converter

MINOR: sample: improve error reporting on missing arg to strcmp() converter

Calling the strcmp() converter with no argument yields this strange error:

  [ALERT]    (31439) : parsing [test.cfg:3] : error detected in frontend 'f' while parsing 'http-request redirect' rule : failed to parse sample expression <src,strcmp]> : invalid args in converter 'strcmp' : failed to register variable name ''.

This is because the vars name check tries to see if it can create such a
variable having an empty name. Let's at least make a special case of the
missing argument. Now we can read a more explicit:

  [ALERT]    (31655) : parsing [test.cfg:3] : error detected in frontend 'f' while parsing 'http-request redirect' rule : failed to parse sample expression <src,strcmp]> : invalid args in converter 'strcmp' : missing variable name.

This was done for secure_strcmp() as well.