Scripts: make shellcheck a bit happier on verify.sh

I am still getting:
```
In scripts/debian/verify.sh line 47:
  && echo 'OK: ALL WORKED FINE!' || (echo 'KO: ERROR!!!' && exit 1)
  ^-- SC2015 (info): Note that A && B || C is not if-then-else. C may run when A is true.

For more information:
  https://www.shellcheck.net/wiki/SC2015 -- Note that A && B || C is not if-t...
```

scripts/debian/verify.sh

@@ -13,7 +13,7 @@ while [[ "$#" -gt 0 ]]; do case $1 in
   *) echo "Unknown parameter passed: $1"; exit 1;;
 esac; shift; done
 
-if [ -z $PACKAGE ]; then
+if [ -z "${PACKAGE}" ]; then
   echo "No package defined. exiting.."; exit 1;
 fi