Fix false nf-test failure on empty stderr

[sapunyangkut]

What changed

• only append the interactive nf-test failed error when stderr is non-empty
• preserve the existing snapshot-difference and --no-prompts behavior
• add a focused regression test for empty stderr, ordinary stderr, and snapshot-difference stderr

The previous bare else was attached to the snapshot-difference check, so a successful interactive nf-test run with empty stderr still appended an error. Snapshot stability runs the command twice, producing the two false failures reported in the issue.

Fixes #4327.

Validation

• python -m pytest tests/components/test_components_snapshot_test.py -k display_nftest_output_stderr -q (3 passed)
• Ruff 0.15.12 check and format check on both changed files
• python -m py_compile on both changed Python files
• git diff --check

PR checklist

• The change is based on and targets dev
• The change includes a focused regression test
• The commit includes a DCO sign-off

Automation disclosure

This small bug fix was prepared and tested with OpenAI Codex automation and independently red-teamed by a separate local agent. The contributor remains responsible for the submitted change; no human review is claimed.