PHP 8.1 issue with preg_match()

Problem/Motivation

If you have a value condition using a regular expression condition, but the field has no value (including for example on the new content form) then it causes an error:

Deprecated function: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated in Drupal\conditional_fields\ConditionalFieldsFormHelper::evaluateDependency() (line 793 of modules/contrib/conditional_fields/src/ConditionalFieldsFormHelper.php).

Steps to reproduce

1. Create an entity reference field which uses a select widget. The field should not be required.
2. Create another field (e.g. a text field).
3. Add a conditional field to control visibility of this 2nd field based on the 1st (entity reference) field.
4. Choose "Condition" = "Value".
5. Choose "Values input mode" = "Regular expression..."
6. Enter a regular expression which will match one of the values in the entity reference field.
7. Edit some content which doesn't have a value for the entity reference field.
8. Save the content.
9. The deprecation appears.

Proposed resolution

It should be able to handle the fact that a field might not have a value.

Now that PHP 8.1 requires string values to string functions, the simplest thing would be to cast the value and this will maintain the PHP 7 behaviour.

Remaining tasks

User interface changes

API changes

Data model changes