resourceloader: Fix undefined $filename in safeFileHash()

resourceloader: Fix undefined $filename in safeFileHash()

Follows-up 28f6d7fbdee.

This was causing versions to stay stuck at their current version.
It worked fine for the initial deployment since the rest of the
definition summary is still fine, but subsequent deploys that changed
only the file were ineffective due to these silently becoming
empty strings.

Thanks to suppressWarnings() we never saw these anywhere in the
tests or in production.

Change-Id: Ib4371255fe970442bcc0041219bce633a7f6f6dd