Fix statsd returning uidmap with empty reports.

We notice devices uploading a bunch of bytes for the uidmap even if
the device is running an empty config, so there are no actual metrics
to report. This hardcodes some logic to skip the inclusion of the
uidmap if there are exactly 0 metrics.

Bug: 79381210
Test: Tested unit-tests on marlin-eng
Change-Id: I96348235341a7faf15ff57d4d1eccac635a3a999