chore: compat PHP 8.1 enlever les deprecated

lib/citeproc-php/CiteProc.php

@@ -235,7 +235,7 @@ class csl_rendering_element extends csl_element {
     $text = '';
     $text_parts = array();
 
-    $delim = $this->delimiter;
+    $delim = isset($this->delimiter) ? $this->delimiter : '';
     foreach ($this->elements as $element) {
       $text_parts[] = $element->render($data, $mode);
     }
@@ -1208,7 +1208,7 @@ class csl_group extends csl_format{
     if ($variables  && !$have_variables ) return; // there has to be at least one other none empty value before the term is output
     if (count($text_parts) == $terms) return; // there has to be at least one other none empty value before the term is output
 
-    $delimiter = $this->delimiter;
+    $delimiter = isset($this->delimiter) ? $this->delimiter : '';
     $text = implode($delimiter, $text_parts); // insert the delimiter if supplied.
 
 
@@ -1232,7 +1232,8 @@ class csl_layout extends csl_format {
       $parts[] = $element->render($data, $mode);
     }
 
-    $text = implode($this->delimiter, $parts);
+    $delimiter = isset($this->delimiter) ? $this->delimiter : '';
+    $text = implode($delimiter, $parts);
 
     if ($mode == 'bibliography') {
       return $this->format($text);