fix: no ENOENT when nothing retrieved to specific directory

[shetzel]

What does this PR do?

When no metadata was retrieved and the --output-dir flag was used, fixes the ENOENT error that was thrown.

What issues does this PR fix or reference?

@W-17355528@

[WillieRuemmele]

QA notes

---

before

 ➜  sf project retrieve start --metadata ApexClass:NON --output-dir abc -o steve

 ────────────── Retrieving Metadata ──────────────

 Retrieving v60.0 metadata from [email protected] using the v62.0 SOAP API

 ✔ Preparing retrieve request 7ms
 ✔ Sending request to org 156ms
 ✔ Waiting for the org to respond 669ms
 ✔ Done 0ms

 Status: Succeeded
 Elapsed Time: 833ms

Error (ENOENT): ENOENT: no such file or directory, stat '/Users/william.ruemmele/projects/scratches/dreamhouse-lwc/abc/main/default'

after ✅

 ➜  ../../oss/plugin-deploy-retrieve/bin/run.js project retrieve start --metadata ApexClass:NON --output-dir abc -o steve

 ────────────── Retrieving Metadata ──────────────

 Retrieving v60.0 metadata from [email protected] using the v62.0 SOAP API

 ✔ Preparing retrieve request 8ms
 ✔ Sending request to org 119ms
 ✔ Waiting for the org to respond 635ms
 ✔ Done 0ms

 Status: Succeeded
 Elapsed Time: 772ms

 ›   Warning: Nothing retrieved

Warnings
┌────────────────────────┬────────────────────────────────────────────────────────┐
│ File                   │ Problem                                                │
├────────────────────────┼────────────────────────────────────────────────────────┤
│ unpackaged/package.xml │ Entity of type 'ApexClass' named 'NON' cannot be found │