registration: Allow the mtime of extension.json files to be zero

registration: Allow the mtime of extension.json files to be zero

When creating Docker images of MediaWiki using the Bazel build system, I
noticed that I'm not able to load any extensions. This is due to the
fact that Bazel always generates container layers with mtimes of files
set to 1970-01-01 for determinism/reproducibility.

Relax the check a bit to only fail when the mtime is false, which
happens when filemtime() fails.

Bug: T196672
Change-Id: Ieaeb3113a7d9c44f29cca2d062c5bb11ebeada0d
(cherry picked from commit d7d5d3c82a84338bd2669284427193ea7a80bdbc)