Remove AtomicReference from StatsdMeterRegistryTest #6311
Description
It fails the NullAway checks:
> Task :micrometer-registry-statsd:compileTestJava
/home/circleci/micrometer/implementations/micrometer-registry-statsd/src/test/java/io/micrometer/statsd/StatsdMeterRegistryTest.java:145: error: [NullAway] dereferenced expression registryReference.get() is @Nullable
registryReference.get().close();
^
(see http://t.uber.com/nullaway )