Package rehearsal: omit empty optional pip arguments

[jeffhuber]

Problem

The package-install rehearsal passes empty optional pip values as real argv entries when no candidate index or find-links directory is configured. A successful v1.0.9 TestPyPI rehearsal therefore emitted warnings for an empty extra index and empty find-links location.

The install and bounded propagation retry still passed, so this is diagnostic noise rather than a release blocker.

Ask

• Omit optional pip flags entirely when their value is empty.
• Preserve the closed TestPyPI candidate flow and production dependency index behavior.
• Add focused argv tests for absent and present optional values.
• Keep command diagnostics metadata-only; do not persist raw pip output.

Found while qualifying v1.0.9 under #729.