core: return {} if cached synced folders file missing [GH-3550]

A missing synced folders cache indicates an empty cache, not a failure
that should be handled by the caller. The cache file is missing from
data dirs created by an earlier version of Vagrant.

Fixes #3550.
Merge pull request #3551 from benesch/3550

core: return {} if cached synced folders file missing [GH-3550]