// Redirect user if no avatar
function buddypress_redirect_if_no_avatar($redirect_url,$request_url,$user) {

  $avatar = bp_core_fetch_avatar( array( 'item_id' => $user->ID, 'no_grav' => true, 'html'=>false) );

  $pos = strpos($avatar, 'mystery-man');
  if ($pos === false) { return "/";}
  else {
    $redirect_url = bp_core_get_user_domain($user->ID)."/profile/change-avatar/";
    return $redirect_url;
  }

}

add_filter("login_redirect","buddypress_redirect_if_no_avatar",100,3);