fix(undo): record NULL version_id when inline capture is empty

[mfreed]

ExecuteUndoTransaction's Step 3 inserts a type='undo' log entry for every affected file, with version_id pointing at the row that the archive BEFORE trigger wrote during Step 1's DELETE or Step 2's UPSERT (captured inline immediately after each mutation). The capture invariant assumes the archive trigger fires -- which it does in any properly-configured history-enabled workspace.

If the trigger is missing or disabled (a malformed workspace), the captured-version_id map ends up with no entry for the affected file and Step 3 reads back the empty string. The previous code passed that empty string as a UUID parameter, which PG rejected with the cryptic error: ERROR: invalid input syntax for type uuid: "" (SQLSTATE 22P02). The entire undo transaction rolled back.

Convert the empty string to a SQL NULL (via a nil interface{}) before the INSERT, so the transaction completes successfully and records the undo entry as "no captured snapshot." NULL is already a legitimate version_id (type='create' log entries record NULL too -- there is no before-state for a fresh row), and downstream undo-of-undo dispatch already returns a clear "cannot undo operation: no version_id" error when it encounters one. The defensive path costs one small helper applied in both the DELETE and RESTORE Step 3 loops.

To keep the workspace-inconsistency signal loud despite the transaction succeeding, emit a logging.Warn from the helper with the affected file_id, the branch (delete vs restore), and a hint to verify the archive trigger exists on the source table. Operators investigating the warning find the malformed workspace; the undo itself isn't blocked by the missing trigger.

Updates TestExecuteUndoTransaction_DeleteOnlyUndo's leading comment to describe the dual purpose it now serves: the original empty- RestoreFileIDs branch guard, plus the new defensive NULL-version_id path. The test passes without any trigger installation, matching the no-trigger scenario the production fix targets.