Properly handle an empty MRU file

plugin/mru.vim

@@ -152,7 +152,10 @@ func! s:MRU_LoadList() abort
   " start with an empty list.
   if filereadable(s:MRU_File)
     let s:MRU_files = readfile(s:MRU_File)
-    if s:MRU_files[0] =~# '^\s*" Most recently edited files in Vim'
+    if empty(s:MRU_files)
+      " empty MRU file
+      let s:MRU_files = []
+    elseif s:MRU_files[0] =~# '^\s*" Most recently edited files in Vim'
       " Generated by the previous version of the MRU plugin.
       " Discard the list.
       let s:MRU_files = []