Took me a little while to get this syntax because bash is super annoying with it’s syntax being different from every other reasonable programming language.
Posting this example so I never have to find it again.
VARIABLE_BOOLEAN="TRUE"
if [[ -z $VARIABLE_BOOLEAN ]] || [[ ! -z $VARIABLE_BOOLEAN && $VARIABLE_BOOLEAN == 'TRUE' ]]; then
echo "variable true or null"
fi