Fix bug in dropwizard configuration

jaeger-dropwizard/src/main/java/com/uber/jaeger/dropwizard/Configuration.java

@@ -41,7 +41,7 @@ public Configuration(
       @JsonProperty("sampler") SamplerConfiguration samplerConfig,
       @JsonProperty("reporter") ReporterConfiguration reporterConfig) {
     super(serviceName, samplerConfig, reporterConfig);
-    this.disable = disable;
+    this.disable = disable == null ? false : disable;
   }
 
   @Override

jaeger-dropwizard/src/test/java/com/uber/jaeger/dropwizard/ConfigurationTest.java

@@ -0,0 +1,11 @@
+package com.uber.jaeger.dropwizard;
+
+import org.junit.Test;
+
+public class ConfigurationTest {
+
+  @Test
+  public void testInstantiableWithNulls() throws Exception {
+    new Configuration("serviceName", null, null, null);
+  }
+}