Fix NullPointerException in ScopeImpl.close (#29)

* Fix NullPointerException in scope.close

* checkstyle

core/src/main/java/com/uber/m3/tally/ScopeImpl.java

@@ -172,8 +172,10 @@ public void close() {
         // all metrics.
         reportLoopIteration();
 
-        // Now that all metrics should be known to the reporter, close the reporter
-        reporter.close();
+        if (reporter != null) {
+            // Now that all metrics should be known to the reporter, close the reporter
+            reporter.close();
+        }
     }
 
     /**

core/src/test/java/com/uber/m3/tally/ScopeImplTest.java

@@ -22,6 +22,7 @@
 
 import com.uber.m3.util.Duration;
 import com.uber.m3.util.ImmutableMap;
+
 import org.junit.Test;
 
 import java.util.HashMap;
@@ -109,6 +110,16 @@ public void close() {
         assertEquals(123, reporter.nextGaugeVal(), EPSILON);
     }
 
+    @Test
+    public void closeWithoutReporter() throws ScopeCloseException {
+        try (Scope scope = new RootScopeBuilder().reportEvery(Duration.ofMinutes(1))) {
+            // Create a gauge, update it, and let the AutoCloseable interface
+            // functionality close the scope right away.
+            Gauge shortLifeGauge = scope.gauge("shortLifeGauge");
+            shortLifeGauge.update(123);
+        }
+    }
+
     @Test
     public void subscopes() {
         final int REPORT_INTERVAL_MILLIS = 10;