fix: treat empty environement variables as unset (#6929)
### Description
Fixes #6921
Setting `TURBO_API` env var to an empty string results in a panic:
```
[0 olszewski@chriss-mbp] /Users/olszewski/code/vercel/turborepo $ TURBO_API= turbo_dev build --filter=docs --output-logs new-only
• Packages in scope: docs
• Running build in 1 packages
• Remote caching enabled
@turbo/workspaces:build: cache hit (outputs already on disk), suppressing logs 899e29b77bbaa6e9
docs:rss: cache hit (outputs already on disk), suppressing logs 9991721850b37e8a
docs:schema: cache hit (outputs already on disk), suppressing logs 6048fc4f56a96905
@turbo/gen:build: cache hit (outputs already on disk), suppressing logs b8021fbfb2521690
docs:build: cache hit (outputs already on disk), suppressing logs 7f1f00f669ffea14
Tasks: 5 successful, 5 total
Cached: 5 cached, 5 total
Time: 495ms >>> FULL TURBO
Oops! Turbo has crashed.
A report has been written to /var/folders/3m/rxkycvgs5jgfvs0k9xcgp6km0000gn/T/report-9ec6c539-2888-430d-9f52-ecd91cf18bc9.toml
Please open an issue at https://github.com/vercel/turbo/issues/new/choose and include this file
WARNING failed to close spaces client
```
This is due to us expecting Go-like behavior where the empty string is
the none value. We can achieve this behavior by mapping `Some("")` to
`None`. I think this is a valid change for the various string type
config options as the empty string is never a valid "present" value for
these fields.
### Testing Instructions
Added unit test to verify that we now treat empty set environment
variables as `None`
Quick spot check that this no longer crashes and uses the default API
instead of `""`:
```
[0 olszewski@chriss-mbp] /Users/olszewski/code/vercel/turborepo $ TURBO_API= turbo_dev build --filter=docs --output-logs new-only
• Packages in scope: docs
• Running build in 1 packages
• Remote caching enabled
@turbo/workspaces:build: cache hit (outputs already on disk), suppressing logs 899e29b77bbaa6e9
docs:schema: cache hit (outputs already on disk), suppressing logs 6048fc4f56a96905
@turbo/gen:build: cache hit (outputs already on disk), suppressing logs b8021fbfb2521690
docs:rss: cache hit, suppressing logs 9991721850b37e8a
docs:build: cache hit (outputs already on disk), suppressing logs 7f1f00f669ffea14
Tasks: 5 successful, 5 total
Cached: 5 cached, 5 total
Time: 502ms >>> FULL TURBO
Run: https://vercel.com/teams/vercel/repos/turbo-monorepo/runs/space_run_EfSNkNZDvtznuFbWhHGfbv1a
```
Closes TURBO-2004
