_wp_json_convert_string type mismatch: returns string on success; false on failure

[hellofromTonya]

The private function _wp_json_convert_string (which converts a string to UTF-8) also returns false, though its @return specifies only string.

How? ​`mb_convert_encoding` returns a string on success and a false on failure.

Is this a bug or docs fix? It's both.

Reviewing its context, it's used by _wp_json_sanity_check to convert IDs and data value. The IDs are the problem. How so? Each ID is used as either a key or project for referencing the data value.

What happens if the conversion to UTF-8 fails? The key or property would either be an empty string or false. That's problematic as it can cause unexpected JSON encoding results.

For example, this ​3v4l snippet simulates the convert encoding failure to view the results through the JSON encoding process. Notice how the returned JSON is invalid when it fails. (Note: This snippet is forcing a failure.)

[desrosj]

#57172 was marked as a duplicate.

Trac Ticket: Core-53023

## Summary

This pull request addresses a critical issue in the _wp_json_convert_string function, which is responsible for converting strings to UTF-8 for safe JSON encoding. The previous implementation did not handle conversion failures appropriately, potentially leading to invalid JSON output. This update ensures that the function consistently returns a string, mitigating unexpected behavior in subsequent JSON encoding processes.

## Changes Made

• Error Handling Improvement: If mb_convert_encoding fails and returns false, the function now explicitly returns an empty string. This change prevents invalid keys or properties in JSON data.

• Code Clarity: The logic for determining the string encoding has been streamlined to enhance readability and maintainability.

• Documentation Update: The function's PHPDoc has been revised to clarify that the function will return an empty string if conversion fails, ensuring alignment between documentation and behavior.

## Implications:

• This update enhances the robustness of the JSON encoding process by ensuring that all outputs from _wp_json_convert_string are valid strings.

• Developers using this function can expect more predictable behavior, as the risk of generating invalid JSON due to conversion errors has been significantly reduced.