Remove redundant null checks from Stats class.
Remove redundant null checks from Stats class.

This change should have been included in #468.

sebright/opencensus-java,bogdandrutu/instrumentation-java,bogdandrutu/opencensus-java,sebright/opencensus-java,census-instrumentation/opencensus-java,bogdandrutu/opencensus-java,sebright/instrumentation-java,bogdandrutu/opencensus-java,sebright/opencensus-java,songy23/instrumentation-java,bogdandrutu/instrumentation-java,sebright/instrumentation-java,census-instrumentation/opencensus-java,songy23/instrumentation-java,census-instrumentation/opencensus-java,songy23/instrumentation-java