ConfigUtil: Annotate getDuration as @nullable

getDuration can return null if the value is not configured and null is
given for the defaultValue (which is already annotated as @nullable).

Change-Id: Iebf59b4f1070eabca7ef6f61e6c51a748123d343

Author: dpursehouse

java/com/google/gitiles/ConfigUtil.java

@@ -39,6 +39,7 @@ public class ConfigUtil {
    * @param defaultValue value to use when the value is not assigned.
    * @return a standard duration representing the time read, or defaultValue.
    */
+  @Nullable
   public static Duration getDuration(
       Config config,
       String section,