[D7 PHP 8.1] strlen(): Passing null to parameter #1 ($string) of type string is deprecated in theme_file_upload_help()

Problem/Motivation

We are experiencing a deprecated function warning after migration to PHP 8.1:

Deprecated function: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in theme_file_upload_help() (line 946 of xxx\modules\file\file.field.inc).

PHP: 8.1
Database: PostgreSQL 11 / MySQL 8 (problem is present regardless of database type)
Drupal: 7.84

The problem appears to be in this part, where $description variable "empty check" is done by strlen() function:

  $description = $variables['description'];
  $upload_validators = $variables['upload_validators'];

  $descriptions = array();

  if (strlen($description)) {
    $descriptions[] = $description;
  }

Steps to reproduce

Issue can be reproduced on clean Drupal 7.84 install:

1. Install Drupal 7.84 with standard profile
2. Edit field_image in "article" content type (admin/structure/types/manage/article/fields/field_image) - set "Number of values" to more than 1 (2, 3, ... or unlimited)
3. Go to "Add content" - "Article" (node/add/article)
4. Deprecated warning appears

Proposed resolution

Replace / supplement the strlen() check with an additional !empty() function.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet