An empty string `""` and `undefined` — are definitely different values, but both
are **falsey**. But the snippet you provided here can't differentiate them. So,
even if the variable **is set, but its value is an empty string** (it is legal) your
code will behave like the variable **is not set**. 

Now fixed.

from https://stackoverflow.com/a/52436493/1115187

core: Fix the check `variable is set`