Remove null check

Author: xiepuhuan

instrumentation/grpc-1.6/library/src/main/java/io/opentelemetry/instrumentation/grpc/v1_6/GrpcRequestGetter.java

@@ -31,7 +31,7 @@ public String get(@Nullable GrpcRequest request, String key) {
 
   @Override
   public Iterator<String> getAll(@Nullable GrpcRequest request, String key) {
-    if (request == null || request.getMetadata() == null) {
+    if (request == null) {
       return emptyIterator();
     }