addess review comments

Author: laurit

instrumentation/restlet/restlet-2.0/library/src/main/java/io/opentelemetry/instrumentation/restlet/v2_0/internal/RestletHeadersGetter.java

@@ -12,6 +12,7 @@
 import java.lang.invoke.MethodHandles;
 import java.lang.invoke.MethodType;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.Map;
 import java.util.concurrent.ConcurrentMap;
@@ -61,7 +62,7 @@ public String get(Request carrier, String key) {
   public Iterator<String> getAll(Request carrier, String key) {
     Series<?> headers = getHeaders(carrier);
     return headers == null
-        ? null
+        ? Collections.emptyIterator()
         : Arrays.asList(headers.getValuesArray(key, /* ignoreCase= */ true)).iterator();
   }
 

instrumentation/spring/spring-webflux/spring-webflux-5.3/library/src/main/java/io/opentelemetry/instrumentation/spring/webflux/v5_3/WebfluxTextMapGetter.java

@@ -5,8 +5,9 @@
 
 package io.opentelemetry.instrumentation.spring.webflux.v5_3;
 
+import static java.util.Collections.emptyIterator;
+
 import io.opentelemetry.context.propagation.internal.ExtendedTextMapGetter;
-import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import javax.annotation.Nullable;
@@ -32,9 +33,9 @@ public String get(@Nullable ServerWebExchange exchange, String key) {
   @Override
   public Iterator<String> getAll(@Nullable ServerWebExchange exchange, String key) {
     if (exchange == null) {
-      return Collections.emptyIterator();
+      return emptyIterator();
     }
     List<String> list = exchange.getRequest().getHeaders().get(key);
-    return list != null ? list.iterator() : Collections.emptyIterator();
+    return list != null ? list.iterator() : emptyIterator();
   }
 }