system-helper: Don't pass NULL for progress

When calling into FlatpakDir apis that take an
OstreeProgress, always pass a no-op instance,
never NULL.

Closes: #2468
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f