eval/encode: Fix writing strings starting with NL to list

Error [found][1] by oni-link.

[1]: https://github.com/neovim/neovim/pull/4131/files#r52239384

test/unit/eval/helpers.lua

@@ -0,0 +1,72 @@
+local helpers = require('test.unit.helpers')
+
+local cimport = helpers.cimport
+local to_cstr = helpers.to_cstr
+local ffi = helpers.ffi
+local eq = helpers.eq
+
+local eval = cimport('./src/nvim/eval.h', './src/nvim/eval_defs.h')
+
+local null_string = {[true]='NULL string'}
+local null_list = {[true]='NULL list'}
+local type_key = {[true]='type key'}
+local list_type = {[true]='list type'}
+
+local list = function(...)
+  local ret = ffi.gc(eval.list_alloc(), eval.list_unref)
+  eq(0, ret.lv_refcount)
+  ret.lv_refcount = 1
+  for i = 1, select('#', ...) do
+    local val = select(i, ...)
+    local typ = type(val)
+    if typ == 'string' then
+      eval.list_append_string(ret, to_cstr(val))
+    elseif typ == 'table' and val == null_string then
+      eval.list_append_string(ret, nil)
+    elseif typ == 'table' and val == null_list then
+      eval.list_append_list(ret, nil)
+    elseif typ == 'table' and val[type_key] == list_type then
+      local itemlist = ffi.gc(list(table.unpack(val)), nil)
+      eq(1, itemlist.lv_refcount)
+      itemlist.lv_refcount = 0
+      eval.list_append_list(ret, itemlist)
+    else
+      assert(false, 'Not implemented yet')
+    end
+  end
+  return ret
+end
+
+local lst2tbl = function(l)
+  local ret = {[type_key]=list_type}
+  if l == nil then
+    return ret
+  end
+  local li = l.lv_first
+  -- (listitem_T *) NULL is equal to nil, but yet it is not false.
+  while li ~= nil do
+    local typ = li.li_tv.v_type
+    if typ == eval.VAR_STRING then
+      str = li.li_tv.vval.v_string
+      if str == nil then
+        ret[#ret + 1] = null_string
+      else
+        ret[#ret + 1] = ffi.string(str)
+      end
+    else
+      assert(false, 'Not implemented yet')
+    end
+    li = li.li_next
+  end
+  return ret
+end
+
+return {
+  null_string=null_string,
+  null_list=null_list,
+  list_type=list_type,
+  type_key=type_key,
+
+  list=list,
+  lst2tbl=lst2tbl,
+}