CLI: emit [] on stdout when site list --format=json finds no sites

[nightnei]

How AI was used in this PR

Claude wrote the fix and the corresponding test. I reviewed and tested locally.

Proposed Changes

NIT: studio site list --format=json returned early with no stdout output when zero sites were configured, so a scripted consumer received empty input and errored trying to parse it. Now the command emits [], matching the contract of the non-empty case.

Testing Instructions

0. Visual review should suffice, but:
1. npm run cli:build
2. Remove all sites
3. node apps/cli/dist/cli/main.mjs site list --format=json
4. Assert that you see an empty array - []
5. node apps/cli/dist/cli/main.mjs site list
6. Assert that no regressions and you still see only ✔ No sites found

[wpmobilebot]

📊 Performance Test Results

Comparing e89bc27 vs trunk

app-size

Metric	trunk	e89bc27	Diff	Change
App Size (Mac)	1483.73 MB	1483.73 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	e89bc27	Diff	Change
load	1798 ms	1931 ms	+133 ms	🔴 7.4%

site-startup

Metric	trunk	e89bc27	Diff	Change
siteCreation	8084 ms	8087 ms	+3 ms	⚪ 0.0%
siteStartup	4961 ms	4949 ms	12 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[fredrikekelund]

Makes sense 👍