Fix potential error when deleting remote files in sync manager

[kj-9]

Summary

Fix sync error when deleting files on Android and syncing with macOS

Issue

Closes #21

When deleting files on Android and then syncing with github repo pushed from macOS, a sync error occurs as described in the linked issue.

Notes

There may be potential to remove the .sha = null setting entirely, but the side effects are unclear for me.

[dedestem]

This isn't only with MacOS.

Because i also get this when syncing between linux & android.

However i hope that this gets accepted.

Thank you for thanking your time to fix it (:

[dbarnett]

+1 hoping this gets merged!

Note there's a parallel error about "path" vs "sha" that I believe comes from calling downloadFile on undefined, but we could treat that separately and file a new issue for it if needed. It's not quite as common as the "undefined (setting sha)" one.

[seeReadCode]

@silvanocerza anything I can do to help get this PR merged? thanks

[dbarnett]

FWIW I've moved to the FIT plugin instead, which is very similar but I found slightly less buggy. It does still have a few bugs like this, but I'm a co-maintainer on FIT now and have some changes nearly finished to finally fix it properly there.