Update login.tpl

Make it possible to make a permalink with username pre-filled in.
there.. && !empty

and !empty
rather !isset || empty

if NOT set OR is empty, then placeholder, else value.
tweaks to t13's patch

* Always have the placeholder
* Rename parameter to something not likely to conflict anywhere
* Escaping for XSS safety
Merge branch 't13_patch-1'
Merge branch 'master' of https://github.com/enwikipedia-acc/waca