This will return true if a variable is unset or set to the empty string ("").

if [ -z "$VAR" ];

This will return true if a variable is unset or set to the empty string ("").

if [ -z "${VAR}" ];

This will return true if a variable is unset.

if [ -z "$VAR" ];

This will return true if a variable is unset or set to the empty string ("").

if [ -z "$VAR" ];