Removed the - signs from default spring var resolution

with the current setup we're using the MDC's default interpolation as a default for spring var resolution (which makes no sense)
with this change we're removing the dashes

fixes gh-1396

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/autoconfig/TraceEnvironmentPostProcessor.java

@@ -49,7 +49,7 @@ public class TraceEnvironmentPostProcessor implements EnvironmentPostProcessor {
 		if (Boolean
 				.parseBoolean(environment.getProperty("spring.sleuth.enabled", "true"))) {
 			map.put("logging.pattern.level", "%5p [${spring.zipkin.service.name:"
-					+ "${spring.application.name:-}},%X{X-B3-TraceId:-},%X{X-B3-SpanId:-},%X{X-Span-Export:-}]");
+					+ "${spring.application.name:}},%X{X-B3-TraceId:-},%X{X-B3-SpanId:-},%X{X-Span-Export:-}]");
 		}
 		addOrReplace(environment.getPropertySources(), map);
 	}