fix: extra blank lines in gradle output

[sebastianbarrozo]

When running rtk gradle, the filtered output includes extra blank lines:

1. Leading blank lines at the start of output
2. Extra newline when stderr is empty (stdout + '\n' + empty stderr)

Expected: Clean output without leading/trailing blank lines.
Actual: Extra blank lines appear in filtered output.

[pszymkowiak]

[w] wshm · Automated triage by AI

🔍 Automated Triage

🐛 Category	bug
🟢 Priority	low
🎯 Confidence	92%

Summary

Gradle output filter produces extra blank lines from leading whitespace and empty stderr concatenation; PR #381 already in progress.

This looks like a simple fix. Only repo maintainers can trigger auto-fix.

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Triaged automatically by wshm · This is an automated analysis, not a human review.