fix(output): avoid blank tee hint lines

[Ha1baraA11]

Summary

• Avoid emitting a leading blank line when a filtered command has no body but does have a tee hint.
• Add a regression test for the shared guarded-emission path.

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk <command> output inspected
• cargo fmt --all -- --check, cargo test --bin rtk, and Clippy pass.
• cargo test --tests has two existing develop-baseline Git-output assertion failures unrelated to this change.

Fixes #4159

[CLAassistant]

All committers have signed the CLA.

[KuSh]

Thanks for this fix. The empty-body case is now handled on develop by #4154 (merged 2026-10-09), which reworked emit_guarded so that with no filtered text the recovery hint is printed alone, without a blank line above it. Develop also carries a regression test for exactly this case (empty_body_with_hint_prints_the_hint_alone in src/core/runner.rs). If I missed something this PR covers, please comment or reopen.