ivy: Fix error on nil input with counsel search

Fix issue syl20bnr#6499: SPC m (spacemacs/search-project-auto-region-or-symbol) on
a blank line causes an error:

    rxt-string->pcre: Wrong type argument: arrayp, nil

* layers/+completion/ivy/funcs.el (spacemacs/counsel-search): Check whether
initial-input is nil in order to avoid calling rxt-quote-pcre on a nil
value.

layers/+completion/ivy/funcs.el

@@ -157,7 +157,7 @@ that directory."
                                               spacemacs--counsel-search-max-path-length)
                                            (length default-directory))))))
        (spacemacs//make-counsel-search-function tool)
-       :initial-input (rxt-quote-pcre initial-input)
+       :initial-input (when initial-input (rxt-quote-pcre initial-input))
        :dynamic-collection t
        :history 'counsel-git-grep-history
        :action #'counsel-git-grep-action