fix: remove %pistol-extra% arguments when not passed to pistol

[lucas-mior]

Although it took long for me to notice it,
PR (#58) introduced a bug which would cause trouble when invoking pistol without extra arguments (and the config expected those).

Basically, it would pass, for example, %pistol-extra3%, literally, instead of the argument (because it wasn't there), instead of just removing it from the arguments array.
The solution, as can be read in the code, is to make the argument an empty string and then, when appending to the other arguments, check if it is empty before doing so.

Note that this is necessary if one is to put multiple %pistol-extra% as in #56 .

[doronbehar]

Sorry it took me a while to answer. This looks good. Thanks for maintaining this part of the code.