Looks like the brackets are fine even in the bracket case Steve mentioned.

@echo off

if NOT DEFINED ABCDEF (
  echo "all good"
)

if NOT DEFINED ABCDE (
  echo "all good2"
)

set ABCDE=

if NOT DEFINED ABCDE (
  echo "sadness"
)

if [%ABCDE%]==[] (
  echo "all good3"
)

set ABCDE=]

if [%ABCDE%] NEQ [] (
  echo "all good4"
)

set ABCDE="]"

if [%ABCDE%] NEQ [] (
  echo "all good5"
)


>demo.bat
"all good"
"sadness"
"all good3"
"all good4"
"all good5"