Pull Request resolves #48123.

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

PlgFieldsSubform::beforePrepareField() calls json_decode($field->value, true) after only checking is_array($field->value). Any subform custom field that has never been filled in has $field->value === null, so json_decode() is called with null, which triggers a PHP 8.1+ deprecation notice:

Deprecated: json_decode(): Passing null to parameter #1 ($json) of type string is deprecated

This adds an early return for null/empty string values, following the same pattern as the existing is_array() guard directly above it.

This PR was drafted with AI assistance (Claude Code) and reviewed/tested by me on a live Joomla 6.1.2 site before submitting, per the Generative AI policy.

Testing Instructions

1. Create a custom field of type Subform on an article/category/etc.
2. View an item where that subform field has never been given a value (empty/null).
3. With error_reporting including deprecation notices (PHP 8.1+), observe the notice is raised.
4. Apply this patch and reload — the notice is gone, page renders identically otherwise.

Actual result BEFORE applying this Pull Request

Deprecated: json_decode(): Passing null to parameter #1 ($json) of type string is deprecated notice shown/logged whenever an unfilled subform field is prepared.

Expected result AFTER applying this Pull Request

No deprecation notice; behavior for non-null values is unchanged.

Link to documentations

Please select:

• No documentation changes for guide.joomla.org needed

• No documentation changes for manual.joomla.org needed