[sushan5140]

Fixes #365.

Summary

• Handle empty $$ placeholder expansions by returning "" instead of raising IndexError.
• Add a regression case verifying that f() rewrites to g().

Testing

• Isolated red/green checks: 5/5 passed.
• Full repository test suite not run locally because the project requires Python 3.14.9 and the available environment lacks the required interpreter and dependencies.
• Awaiting upstream CI verification.

AI assistance was used to prepare the change.

[pjljvandelaar]

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