Fix the test case that the previous commit added

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/widget/base.lua

@@ -337,6 +337,7 @@ local function parse_table(t, leave_empty)
     local attributes, widgets = {}, {}
     for k,v in pairs(t) do
         if type(k) == "number" then
+            if v then
                 -- As `ipairs` doesn't always work on sparse tables, update the
                 -- maximum
                 if k > max then
@@ -344,6 +345,7 @@ local function parse_table(t, leave_empty)
                 end
 
                 widgets[k] = v
+            end
         else
             attributes[k] = v
         end