Fix CF_RELEASE_BRANCH to be handled properly

Current implementation omits CF_RELEASE_BRANCH even if it is set, because
```
[ -z "$CF_RELEASE_BRANCH" ]
```
returns 1.

You can easily confirm this by:

```
export CF_RELEASE_BRANCH=fix-cf-release-branch-handling
[ -z "$CF_RELEASE_BRANCH" ]
echo $?
```

Please fix it.
Thanks in advance.