tag.lua: check name argument to add() is valid

Signed-off-by: Perry Hargrave <perry.hargrave@gmail.com>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/tag.lua.in

@@ -55,8 +55,14 @@ end
 --- Add a tag.
 -- @param name The tag name, a string
 -- @param props The tags properties, a table
--- @return The created tag
+-- @return The created tag or nil if not created.
+-- If the name is a valid string then the tag will be created and each key,value
+-- pair in 'props' table is iterated and applied to the tag. If the 'name'
+-- argument is nil or its string length is zero then the function returns nil.
 function add(name, props)
+    -- abort if the name is not provided
+    if not name or #(tostring(name)) == 0 then return end
+
     local properties = props or {}
     local newtag = capi.tag{name = name}
     newtag.screen = properties.screen or capi.mouse.screen