Changed 2nd parameter ot setenv from '' to nil

Author: ArmCorelin

.gitignore

@@ -1,2 +1,3 @@
 *.elc
 .cask/
+.markdown-preview.html

conda.el

@@ -230,7 +230,7 @@ Set for the lifetime of the process.")
     (mapc (lambda (arg)
             (let ((inhibit-message t))
 	      (message "About to unset %s" arg))
-            (setenv (format "%s" arg) ""))
+            (setenv (format "%s" arg) nil))
           unsets))
  (let ((sets (or (conda-env-params-vars-set params) '())))
     (mapc (lambda (pair)