Lua nils are weird

[rtsisyk]

tarantool> msgpack.NULL or "hello"

---
- null
...

tarantool> nil or "hello"

---
- hello <!-- WTF
...
tarantool> if msgpack.NULL then print('hi') end
hi <!-- WTF

---
...

tarantool> if nil then print('hi') end

---
...

tarantool> msgpack.NULL == true

---
- false
...
tarantool> nil == true

---
- false
...
tarantool> msgpack.NULL == false

---
- false
...

tarantool> nil == false

---
- false
...

tarantool> not msgpack.NULL

---
- false <!-- AHH
...

tarantool> not nil

---
- true
...
tarantool> msgpack.NULL == nil
—-
- true
...

[rtsisyk]

I need tag "DNA bug"

[rtsisyk]

msgpack.NULL can be easily replaced by 0 in the most cases except one:

tarantool> 0 == nil
—-
- false
...

[igormunkin]

Dup of #4227. Closing with the same reason.