code: private String getSignerRegion() {
          String region = getSignerRegionOverride();
          if (region == null) {
              region = clientRegion;
          }
          return region;
      }
docs: Return the region string that should be used for signing requests sent by
  this client. This method can only return null if both of the following
  are true:
  (a) the user has never specified a region via setRegion/configureRegion/setSignerRegionOverride
  (b) the user has specified a client endpoint that is known to be a global S3 endpoint