Review fixes for async-http-client-common-1.8:javaagent (open-telemetry#18102)

Co-authored-by: otelbot <197425009+otelbot@users.noreply.github.com>

Author: otelbot[bot]

instrumentation/async-http-client/async-http-client-common-1.8/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/asynchttpclient/common/v1_8/AsyncHttpClientHelper.java

@@ -23,8 +23,9 @@ public interface AsyncHttpClientHelper {
    * Get the server address (host) from the request.
    *
    * @param request the HTTP request
-   * @return the server address
+   * @return the server address, or {@code null} if the request URI has no host
    */
+  @Nullable
   String getServerAddress(Request request);
 
   /**

instrumentation/async-http-client/async-http-client-common-1.8/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/asynchttpclient/common/v1_8/AsyncHttpClientHttpAttributesGetter.java

@@ -50,6 +50,7 @@ public List<String> getHttpResponseHeader(Request request, Response response, St
   }
 
   @Override
+  @Nullable
   public String getServerAddress(Request request) {
     return helper.getServerAddress(request);
   }