last-fm.lua: don't use empty album names

plugins/degesch/last-fm.lua

@@ -61,8 +61,8 @@ local process = function (buffer, data, action)
 
 	-- Need to make some sense of the XML automatically converted to JSON
 	local text_of = function (node)
-		if type (node) == "table" then return node["#text"] end
-		return node
+		if type (node) ~= "table" then return node end
+		return node["#text"] ~= "" and node["#text"] or nil
 	end
 
 	local name, artist, album