MDL-49293 db: Pass sql_concat_join to sql_concat

lib/dml/mssql_native_moodle_database.php

@@ -1207,11 +1207,7 @@ class mssql_native_moodle_database extends moodle_database {
         for ($n=count($elements)-1; $n > 0 ; $n--) {
             array_splice($elements, $n, 0, $separator);
         }
-        $s = implode(' + ', $elements);
-        if ($s === '') {
-            return " '' ";
-        }
-        return " $s ";
+        return call_user_func_array(array($this, 'sql_concat'), $elements);
     }
 
    public function sql_isempty($tablename, $fieldname, $nullablefield, $textfield) {

lib/dml/sqlsrv_native_moodle_database.php

@@ -1272,12 +1272,7 @@ class sqlsrv_native_moodle_database extends moodle_database {
         for ($n = count($elements) - 1; $n > 0; $n--) {
             array_splice($elements, $n, 0, $separator);
         }
-        $s = implode(' + ', $elements);
-
-        if ($s === '') {
-            return " '' ";
-        }
-        return " $s ";
+        return call_user_func_array(array($this, 'sql_concat'), $elements);
     }
 
     public function sql_isempty($tablename, $fieldname, $nullablefield, $textfield) {