Update file when all persisted jobs are removed.

Make sure we update the persisted file(s) when all persisted jobs are
removed. Previously, we wouldn't touch the file at all when the last
persisted job was removed because there was no job to copy out of the
current job list to write into the file.

Bug: 260147120
Test: atest FrameworksServicesTests:JobStoreTest
Change-Id: I90d28e16d75bbd797997233e0d90ab5145ab72e3