fix: reject empty cache dir path (#7630)
### Description
As mentioned in #6294 we don't want to allow for an empty cache dir to
be set.
This PR now errors if the user passes an empty string as an argument for
`--cache-dir`
### Testing Instructions
Added unit test. Quick manual test:
```
[0 olszewski@chriss-mbp] /Users/olszewski/code/vercel/turborepo $ turbo_dev build --cache-dir=
ERROR invalid value '' for '--cache-dir <CACHE_DIR>': path must not be empty
For more information, try '--help'.
```
Closes TURBO-2534
