Fix b:qf_isLoc for empty location list

Previously with `call setloclist(0, []) | lopen` it would be set to 0.

Author: blueyed

after/ftplugin/qf.vim

@@ -34,7 +34,11 @@ set nobuflisted
 let b:undo_ftplugin .= "| setl wrap< rnu< nu< bl<"
 
 " are we in a location list or a quickfix list?
-let b:qf_isLoc = !empty(getloclist(0))
+if has('patch-7.4.2215')
+    let b:qf_isLoc = getwininfo(win_getid())[0].loclist
+else
+    let b:qf_isLoc = !empty(getloclist(0))
+endif
 
 " customize the statusline
 if exists("g:qf_statusline")