completion/{helm,ivy}: fix stringp error

Cause by the expectation that doom-project-root would never return
nil (which was changed to return nil if not in a valid project, due to
an update upstream, in projectile).

modules/completion/helm/autoload/helm.el

@@ -122,7 +122,7 @@ order.
   (declare (indent defun))
   (require 'helm-ag)
   (helm-ag--init-state)
-  (let* ((project-root (doom-project-root))
+  (let* ((project-root (or (doom-project-root) default-directory))
          (directory (or in project-root))
          (default-directory directory)
          (helm-ag--default-directory directory)