[core] Fix: removes nils from layers list

Spacemacs allows to define layers declaratively like so:

dotspacemacs-configuration-layers
`(,(when (eq system-type 'darwin) 'osx))

The problem - in Linux that would add a nil element into the list, which then
makes it unable to run dotspacemacs/sync-configuration-layers <SPC f e R>, the
tests won't pass.

CHANGELOG.develop

@@ -664,6 +664,8 @@ Other:
   - Avoid unnecessary packages installation of tern layer
   - Fixed terminal mode line indicator overlapping (thanks to duianto)
   - Fixed broken --timed-requires and --adv-timers under Emacs 27 and above (thanks to Ying Qu)
+  - Removed nils from =dotspacemacs-configuration-layers= when running tests
+    (thanks to Ag Ibragimov)
 - Other:
   - New function =configuration-layer/message= to display message in
     =*Messages*= buffer (thanks to Sylvain Benner)

core/core-dotspacemacs.el

@@ -828,7 +828,7 @@ error recovery."
                           "exists in filesystem" "path")
     (setq dotspacemacs-configuration-layers
           (mapcar (lambda (l) (if (listp l) (car l) l))
-                  dotspacemacs-configuration-layers))
+                  (remove nil dotspacemacs-configuration-layers)))
     (spacemacs//test-list 'configuration-layer/get-layer-path
                           'dotspacemacs-configuration-layers
                           "can be found" "layer")