Use nextSpan instead of joinSpan

To account for the case when `contextOrFlags.context() == null`

Author: simao

core/src/main/scala/brave/play/ZipkinTraceServiceLike.scala

@@ -1,5 +1,5 @@
 /**
- * Copyright 2017-2020 The OpenZipkin Authors
+ * Copyright 2017-2021 The OpenZipkin Authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
  * in compliance with the License. You may obtain a copy of the License at
@@ -183,7 +183,8 @@ trait ZipkinTraceServiceLike {
       (carrier: A, key: String) => getHeader(carrier, key).orNull
     ).extract(headers)
 
-    tracer.joinSpan(contextOrFlags.context())
+    Option(contextOrFlags.context())
+      .map(tracer.joinSpan).getOrElse(tracer.nextSpan(contextOrFlags))
   }
 
   /**