Guard ExcludedEncodeContentSC filter entry points against non-string input

[MikeWard0321]

Summary

ExcludedEncodeContentSC::changeContentBeforeEncoderModify() and ::changeContentAfterEncoderModify() are registered directly as WordPress filter callbacks, so they receive whatever value the filter carries — which is not always a string. Neither checks before passing that value to preg_match_all() / preg_replace_callback().

Its sibling EncodeContentSC is attached to the same filters by the same ShortCodesService methods and already guards both of its entry points with exactly this check. This PR applies the same guard to ExcludedEncodeContentSC so the two classes behave consistently on the hooks they share.

The inconsistency

ShortCodesService registers both classes together:

public function addActionsBeforeModify($hook, $priority = 1)
{
    add_filter($hook, array($this->encode, 'changeContentBeforeEncoderModify'), $priority);
    add_filter($hook, array($this->shortcode_to_exclude, 'changeContentBeforeEncoderModify'), $priority);
}

Class	changeContentBeforeEncoderModify()	changeContentAfterEncoderModify()
EncodeContentSC	guards (line 100)	guards (line 145)
ExcludedEncodeContentSC	no guard (line 77)	no guard (line 102)

ExcludedEncodeContentSC::processTitleString() (line 879) also already handles the null case explicitly, so the class does anticipate non-string input elsewhere — just not at the two filter entry points.

Observed behaviour

With a null filter value, changeContentBeforeEncoderModify() passes it to isShortcodeInsideHtmlAttribute() → preg_match_all() at line 521, then to preg_replace_callback() at line 85:

PHP Deprecated: preg_match_all(): Passing null to parameter #2 ($subject) of type string is deprecated
  in .../ContactsEncoder/Shortcodes/ExcludedEncodeContentSC.php:521
PHP Deprecated: preg_replace_callback(): Passing null to parameter #3 ($subject) of type string is deprecated
  in .../ContactsEncoder/Shortcodes/ExcludedEncodeContentSC.php:85

Both fire on every affected page render. On PHP 9 these become a TypeError rather than a deprecation.

Environment: Spam protection, Anti-Spam, FireWall by CleanTalk 6.85, WordPress 7.0.4, PHP 8.4.24, nginx + PHP-FPM. Line numbers above are against current master.

The change

Adds the existing guard to both entry points — 8 lines, no behaviour change for string input:

if ( ! is_string($content) ) {
    return $content;
}

Returning $content unmodified preserves filter semantics for any non-string value rather than coercing it, matching what EncodeContentSC already does.

Verification

Applied to a production site running 6.85 on PHP 8.4.24. Before: the deprecation pair on every render of an affected page. After: no deprecations across repeated requests to the same pages, with page output and rendered content unchanged.