fix(sync-templates): empty matrix strategy expansion (#7415)

# Description

There is a small error (which slipped through reviews) in matrix
strategy expansion which results in errors like this:
https://github.com/paritytech/polkadot-sdk/actions/runs/13079943579/job/36501002368.

## Integration

N/A

## Review Notes

Need to fix this in master and then rerun it manually against
`stable2412-1`.

Signed-off-by: Iulian Barbu <iulian.barbu@parity.io>