Closes #95: defining local variable and fixing args check

plugins/python/pyclean.fish

 # clean current directory recursively from any .pyc and .pyo files
 function pyclean
-  if test -n $argv
-    set path2CLEAN $argv
-  else
+  set -l path2CLEAN
+
+  if set -q $argv
     set path2CLEAN .
+  else
+    set path2CLEAN $argv
   end
 
   find $path2CLEAN -name "*.py[co]" -type f -delete
-end
\ No newline at end of file
+end