fix: don't crash import when backup has no filters/tasks/triggers array

[7MS8]

Split out of #406 per reviewer request (separate unrelated fixes into their own PRs).

Bug

Importer.createFilterList() (and the equivalent task/trigger readers) call reader.getJSONArray("filters"), which throws JSONException when the key is absent from the JSON. Backups exported by older app versions don't contain a filters array (it was added later), so importing such a backup throws partway through Importer.importJson() and aborts the entire import — remotes get written, but tasks/preferences never do. This matches the symptom reported in review of #406 ("remotes populated but no tasks imported"), which the reviewer confirmed as reproducible.

Fix

Replace getJSONArray() with optJSONArray() for filters, tasks, and trigger, returning an empty list when the key is missing, so a backup missing a newer field still imports the sections that are present instead of aborting entirely.

Reproduction

• Take (or construct) a backup export JSON that has no top-level filters key (e.g. any backup made before filters were added, or a hand-edited JSON with that key removed).
• Import it via Settings → Import.
• Before this fix: import throws a JSONException inside createFilterList(), the whole import aborts — remotes/rclone.conf get written but tasks and other settings are silently never imported.
• After this fix: import completes for all sections present in the backup.

Test plan

• Local build compiles (./gradlew :app:compileOssDebugSources).
• Import a legacy backup JSON without a filters key and confirm tasks/preferences now import correctly.

[scr4bble]

@newhinton I believe this PR is quite useful, and both the issue and the fix are clear and straightforward, so it should require very little testing. Would you have a bit of time to take a look and, if everything seems fine, approve and merge it?