Tentative fix for https://issues.apache.org/jira/browse/JCLOUDS-1643 (#214)

org.jclouds.googlecomputeengine.domain.Warning: Add nullable annotation to code and data attributes

Author: fviale

providers/google-compute-engine/src/main/java/org/jclouds/googlecomputeengine/domain/Warning.java

@@ -26,9 +26,9 @@
 @AutoValue
 public abstract class Warning {
 
-   public abstract String code();
+   @Nullable public abstract String code();
    @Nullable public abstract String message();
-   public abstract List<KeyValuePair> data();
+   @Nullable public abstract List<KeyValuePair> data();
 
    @SerializedNames({"code", "message", "data"})
    public static Warning create(String code, String message, List<KeyValuePair> data){