Fix composite widgets with top level container

When wrapping container widgets to create reusable composite widgets,
`drill` will be called twice on the same widget definition. The first
call happens within the wrapping function and applies the children
widgets fine. The second call happens when the composite widget is used,
but since there are no children widgets defined, the call to
`set_children` sets the existing child to `nil` instead.

Fixes #3213.

Signed-off-by: Lucas Schwiderski <lucas@lschwiderski.de>

lib/wibox/widget/base.lua

@@ -533,6 +533,7 @@ local function drill(ids, content)
         end
     end
 
+    if widgets and max > 0 then
         -- Add all widgets.
         for k = 1, max do
             -- ipairs cannot be used on sparse tables.
@@ -555,10 +556,11 @@ local function drill(ids, content)
                 end
             end
         end
+
         -- Replace all children (if any) with the new ones.
-    if widgets then
         l:set_children(widgets)
     end
+
     return l, id
 end