Changed getiter() to disallow nil values

lume.lua

@@ -51,13 +51,12 @@ local iternil = function()
 end
 
 local getiter = function(x)
-  if x == nil then
-    return iternil
-  elseif isarray(x) then
+  if isarray(x) then
     return ipairs
-  else
+  elseif type(x) == "table" then
     return pairs
   end
+  error("expected table", 3)
 end
 
 local iteratee = function(x)
@@ -327,11 +326,13 @@ function lume.concat(...)
   local rtn = {}
   for i = 1, select("#", ...) do
     local t = select(i, ...)
+    if t ~= nil then
       local iter = getiter(t)
       for k, v in iter(t) do
         rtn[#rtn + 1] = v
       end
     end
+  end
   return rtn
 end