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

[nikital]

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.

Spacemacs is great, keep up the good work :)

Edit: Just noticed the pull request number! Do I get eternal glory now? :P

Edit2:
Steps to reproduce:

1. Start with a clean Spacemacs on develop branch and no .spacemacs.d
2. Upon first start choose spacemacs-base for the distribution and vim for editing style
3. Open the generated config (SPC f e d) and replace helm layer with ivy
4. Restart Spacemacs to load Ivy
5. Invoke ivy-spacemacs-help with SPC h SPC and search for vingar no packages (Or any other layer that doesn't own any packages)
6. With Ivy still open and vinegar selected, use M-o p to open Vinegar's packages.el. You will get Wrong type argument: stringp, vinegar.

[nikital]

@syl20bnr @JAremko Is there any change I can do to make this patch easier to accept?

[sdwolfz]

Not denying that what you did works, but I think it would help if you also provided a way to reproduce this bug in the description, so others can try it out for themselves and give you a 👍.

[nikital]

@SDWolf That's a good point, thanks. Updated the pull request with steps to reproduce.

[syl20bnr]

Good catch, Thank you ! 👍
Cherry-picked into develop branch, you can safely delete your branch.