Make g:startify_custom_header handle empty string

eval() throws errors on empty strings.

autoload/startify.vim

@@ -86,7 +86,9 @@ function! startify#insane_in_the_membrane() abort
     if type(g:startify_custom_header) == type([])
       let g:startify_header = copy(g:startify_custom_header)
     elseif type(g:startify_custom_header) == type('')
-      let g:startify_header = eval(g:startify_custom_header)
+      let g:startify_header = empty(g:startify_custom_header)
+            \ ? []
+            \ : eval(g:startify_custom_header)
     else
       echomsg 'startify: wrong type of value for g:startify_custom_header'
       let g:startify_header = startify#fortune#cowsay()