Using simple comparison is less ambiguous, faster and cleaner.

verify.txt

        isset  is_null ===null  ==null  empty
 null |   F   |   T   |   T   |   T   |   T   |
unset |   F   |   T   |   T   |   T   |   T   |
  ""  |   T   |   F   |   F   |   T   |   T   |
  []  |   T   |   F   |   F   |   T   |   T   |
    0 |   T   |   F   |   F   |   T   |   T   |
false |   T   |   F   |   F   |   T   |   T   |
 true |   T   |   F   |   F   |   F   |   F   |
    1 |   T   |   F   |   F   |   F   |   F   |
   \0 |   T   |   F   |   F   |   F   |   F   |