thegamesdb: Check for empty list
thegamesdb: Check for empty list

Checking if table exists is not enough. If xml field exists,
grl.lua.xml.string_to_table() will include that into the returning
table.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=794504

MikePetullo/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,grilofw/grilo-plugins,grilofw/grilo-plugins