Fix MoCache double-applying load_textdomain_mofile filter

[superdav42]

Summary

• Stop re-applying load_textdomain_mofile filter in MoCache::__construct() — WordPress core already applied it before calling override_load_textdomain
• Add empty-path guard before import_from_file() to prevent PHP 8+ ValueError

Fixes #26

Problem

MoCache::__construct() calls apply_filters('load_textdomain_mofile', $mofile, $domain), but $mofile was already filtered by WordPress core in load_textdomain() before reaching the override_load_textdomain hook. This double-filtering breaks plugins with stateful filter callbacks.

Specifically, Polylang Pro's PLL_Locale_Fallback::load_file returns an empty string on the second pass, causing $this->mofile to be ''. Later, import_from_file('') throws ValueError: Path cannot be empty on PHP 8+.

Reproduction

Drop this mu-plugin into any WordPress site with Docket Cache (MOCACHE enabled), PHP 8+, and any plugin that has a .mo translation file:

<?php
/**
 * Plugin Name: Reproduce Docket Cache MoCache Double-Filter Bug
 */
add_filter('load_textdomain_mofile', function ($mofile, $domain) {
    static $seen = [];
    $key = $domain . '|' . $mofile;
    if (isset($seen[$key])) {
        // Second call — MoCache is re-applying the filter. Return empty to
        // simulate what Polylang Pro's PLL_Locale_Fallback does in practice.
        return '';
    }
    $seen[$key] = true;
    return $mofile;
}, 10, 2);

Any page load that triggers a translation will crash with:

PHP Fatal error: Uncaught ValueError: Path cannot be empty
  in wp-includes/pomo/streams.php

Testing

Tested on a production WordPress 6.9 multisite with PHP 8.2, Polylang Pro, and the affiliate-links plugin:

Scenario	Result
Original MoCache, no workaround	500 (Fatal ValueError)
This fix applied	200
Original MoCache restored, 500 returns	500 (confirms fix was the cause)
Fix re-applied	200

Test plan

• Verify translations still load correctly with MOCACHE enabled
• Verify no crash on PHP 8+ with Polylang Pro active
• Verify cached translations are still served from object cache