Fixed error when base_size is not set

lib/wibox/widget/systray.lua

@@ -66,7 +66,10 @@ function systray:draw(context, cr, width, height)
     local cols = math.ceil(num_entries / rows)
     local bg = beautiful.bg_systray or beautiful.bg_normal or "#000000"
     local spacing = beautiful.systray_icon_spacing or 0
-    local y_offset = ((height - base_size) / 2) - 1
+    local y_offset = 0 
+    if base_size then
+        y_offset = ((height - base_size) / 2) - 1
+    end
 
     if context and not context.wibox then
         error("The systray widget can only be placed inside a wibox.")