MDL-76092 core_backup: Fix PHP8.0 vulnerability convert_params_to_values

backup/util/dbops/backup_structure_dbops.class.php

@@ -72,7 +72,7 @@ abstract class backup_structure_dbops extends backup_dbops {
                     throw new base_element_struct_exception('valueofparamelementnotset', $param->get_name());
                 }
 
-            } else if ($param < 0) { // Possibly one processor variable, let's process it
+            } else if (is_numeric($param) && $param < 0) { // Possibly one processor variable, let's process it
                 // See @backup class for all the VAR_XXX variables available.
                 // Note1: backup::VAR_PARENTID is handled by nested elements themselves
                 // Note2: trying to use one non-existing var will throw exception