Allow statsd to be given empty config.

Statsd clients may want to set an empty config temporarily, so it's
more convenient to allow them to set an empty config instead of
having to use the removeConfig and then having to remember to call
StatsManager#setDataFetchOperation.

Test: Added unit-tests and check they pass on marlin-eng.
Bug: 74997752
Change-Id: I2e762e5ec01e5a2c9a3469fb330b53fefbd734d6