fix(zsh_autocomplete): List files on tab with no completion options

This will simply list the files in the current directory if there are no
auto-completion options available.

With this change, the zsh auto-completion will align with the functionality in
the bash auto-completion file provided.

New functionality: https://asciinema.org/a/EAYRIEVGTGNSS2gCGwSJ4Zw1i

Old functionality: https://asciinema.org/a/BfOZz4BHUGwjXMFptbmDHZocH

Signed-off-by: Ole Petter <ole.orhagen@northern.tech>

autocomplete/zsh_autocomplete

@@ -13,6 +13,8 @@ _cli_zsh_autocomplete() {
 
   if [[ "${opts[1]}" != "" ]]; then
     _describe 'values' opts
+  else
+    _files
   fi
 
   return