feat(lua): make vim.mpack support vim.NIL and vim.empty_dict()

test/functional/lua/mpack_spec.lua

@@ -0,0 +1,23 @@
+-- Test suite for testing interactions with API bindings
+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local eq = helpers.eq
+local exec_lua = helpers.exec_lua
+
+describe('lua vim.mpack', function()
+  before_each(clear)
+  it('can pack vim.NIL', function()
+    eq({true, true, true, true}, exec_lua [[
+      local var = vim.mpack.unpack(vim.mpack.pack({33, vim.NIL, 77}))
+      return {var[1]==33, var[2]==vim.NIL, var[3]==77, var[4]==nil}
+    ]])
+  end)
+
+  it('can pack vim.empty_dict()', function()
+    eq({{{}, "foo", {}}, true, false}, exec_lua [[
+      local var = vim.mpack.unpack(vim.mpack.pack({{}, "foo", vim.empty_dict()}))
+      return {var, vim.tbl_islist(var[1]), vim.tbl_islist(var[3])}
+    ]])
+  end)
+end)