api: Add missing @nullable annotation (#12559)

This simple commit adds a missing but necessary @nullable annotation to
`StatusException.getTrailers()`.

Author: Meijuh

api/src/main/java/io/grpc/StatusException.java

@@ -65,6 +65,7 @@ public final Status getStatus() {
    *
    * @since 1.0.0
    */
+  @Nullable
   public final Metadata getTrailers() {
     return trailers;
   }