Bug: ZDOTDIR is not set to a default value

### What happened?

When the ZDOTDIR variable is not set the installer.sh script is failing due to an unbound variable. 

### What did you expect to happen?

ZDOTDIR is set to a default / fallback value 

### Can you reproduce the problem?

Yes, consistently

### CLI Version

v0.565.0

### Relevant log output

```shell
bash: line 255: ZDOTDIR unbound variable
```