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[ -z "$ENV_VAR" -a -z "$ENV_VAR2" ] has 2 conditions ANDed together using -a switch:

What it means is this:

btw if you're using bash you can refactor this condition to make it bit more succinct:

[[ -z $ENV_VAR && -z $ENV_VAR2 ]]
                If you're not using [[ ]], you should use [ ... ] && [ ... ] instead, see Bash Pitfall #6.
– Benjamin W.
                Jan 7, 2019 at 17:02
                That gets you the man page for the external test/[ command, but in Bash, they're built-ins, the help for which is in help test (or man bash).
– Benjamin W.
                Jan 7, 2019 at 17:05
$ [[ -z "" -a -z "" ]] && echo Hello
bash: syntax error in conditional expression
bash: syntax error near `-a'
If used with single [ it is the "and" from test. If used with [[ it is the file check from bash.
The bash solution:
$ [[ -z "" && -z "" ]] && echo Hello
Hello
For POSIX compatibility, [[ ... && ... ]] is not available, but -a is considered obsolete (and optional) by POSIX, so use two separate [ commands instead.
if [ -z "$ENV_VAR" ] && [ -z "$ENV_VAR2" ]; then