small fixes due to ES instrumentation

dd-java-agent/src/main/resources/otarules.btm

@@ -1,4 +1,4 @@
-# --------------------------------------------------------
+	# --------------------------------------------------------
 # Instrument mongo client
 RULE Mongo Client instrumentation
 CLASS com.mongodb.MongoClientOptions$Builder
@@ -81,7 +81,6 @@ AT ENTRY
 IF currentSpan() == null
 DO
     span = getTracer().buildSpan($2.getClass().getSimpleName())
-        .asChildOf()
         .withTag(io.opentracing.tag.Tags.SPAN_KIND.getKey(), io.opentracing.tag.Tags.SPAN_KIND_CLIENT)
         .start();
 #    decorator.onRequest(span);

dd-trace/src/main/java/com/datadoghq/trace/DDTracer.java

@@ -238,7 +238,7 @@ public class DDTracer implements io.opentracing.Tracer {
         }
 
         public DDTracer.DDSpanBuilder asChildOf(Span span) {
-            return asChildOf(span.context());
+            return asChildOf(span==null? null : span.context());
         }
 
         public DDTracer.DDSpanBuilder asChildOf(SpanContext spanContext) {