fix: normalize nullable notification text

[somethingwithproof]

Notification text columns can be NULL, and PHP 8 makes passing NULL into the string functions a deprecation rather than an empty string.

Normalises the values at read time.

Closes #814.

[Copilot]

Pull request overview

This pull request updates the thold_str_replace() notification-template helper to safely accept a nullable subject (notification text) without relying on PHP 8.1’s deprecated null-to-string coercion, and adds a regression test plus minimal documentation updates.

Changes:

• Allow thold_str_replace()’s $subject parameter to be nullable and normalize null to '' before calling str_replace().
• Add a unit regression that asserts the nullable-subject boundary emits no E_DEPRECATED.
• Document the nullable template-field behavior in README and add a CHANGELOG entry for issue #814.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
thold_functions.php	Makes thold_str_replace() accept ?string $subject and normalizes null to empty string to avoid deprecated coercion.
tests/Unit/TholdStrReplaceTest.php	Adds a regression test capturing/forbidding E_DEPRECATED when the subject is null.
README.md	Documents that unset notification template fields are treated as empty text.
CHANGELOG.md	Records the fix for issue #814.

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