ivy: spacemacs-help: Fix candidate layers with no packages

When a layer doesn't have a package, its name was added to the candidate list as
a symbol instead of a string, breaking actions such as
layer-action-open-packages which expect a string.

layers/+completion/ivy/local/ivy-spacemacs-help/ivy-spacemacs-help.el

@@ -230,7 +230,7 @@
                           layer
                           (propertize "no packages"
                                       'face 'warning))
-                  layer
+                  (symbol-name layer)
                   nil)
             result))
     (sort result (lambda (a b) (string< (car a) (car b))))))