[grilo-plugins] metrolyrics: fix initial check up

[Bastien Nocera <hadess src gnome org>]

commit c62b5524663ea5bccb4a0032ff0ee58bd971648c
Author: Victor Toso <me victortoso com>
Date:   Wed Mar 19 02:05:30 2014 -0300

    metrolyrics: fix initial check up
    
    Artist and Title must be non empty strings.
    Return after callback.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=726677

 src/lua-factory/sources/grl-metrolyrics.lua |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/src/lua-factory/sources/grl-metrolyrics.lua b/src/lua-factory/sources/grl-metrolyrics.lua
index 473bf05..f61d15a 100644
--- a/src/lua-factory/sources/grl-metrolyrics.lua
+++ b/src/lua-factory/sources/grl-metrolyrics.lua
@@ -55,8 +55,10 @@ function grl_source_resolve()
   local artist, title
 
   req = grl.get_media_keys()
-  if not req or not req.artist or not req.title then
+  if not req or not req.artist or not req.title
+    or #req.artist == 0 or #req.title == 0 then
     grl.callback()
+    return
   end
 
   -- Prepare artist and title strings to the url