strlen(): Passing null to parameter #1 ($string) of type string is deprecated

Problem/Motivation

When adding a new key, the following error is thrown, when running PHP 8.1+

Deprecated function: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\key\Plugin\KeyType\EncryptionKeyType->validateKeyValue() (line 124 of modules/contrib/key/src/Plugin/KeyType/EncryptionKeyType.php).

Steps to reproduce

Attempting to add a new key and submit the form at /admin/config/system/keys/add.

Type Settings

Key Type: Encryption

Key Size: 256

Provider Settings

Key Provider: Environment

Environment Variable: EXAMPLE_ENCRYPTION_KEY

Error can occur when:

• The environment variable does not exist or it is empty.
• The selected key size does not match the actual size of the key.

Proposed resolution

Do not pass NULL value as instructed in the error message.