Return nothing gracefully if no sequence is provided

src/imapdomo.lua

@@ -207,6 +207,7 @@ pkg.head = function(num, seq)
     --
     -- Return first *num* elements from sequence
     --
+    if not seq then return seq end
     local result = seq:is_smaller(0)    -- HACK to generate empty sequence
     for idx, value in ipairs(seq) do
         if idx > num then break end