Class to batch update records but make not changes. Useful to trigger a flow to take effect on existing data that writes a new field. Replace !!ObjectType!! with your object, e.g. Contact or Lead or CustomObject__c

EmptyUpdateBatch.cls

global class EmptyUpdateBatch implements Database.Batchable<sObject> {
    global Database.QueryLocator start(Database.BatchableContext BC) {
        String query = 'SELECT Id FROM !!ObjectType!! WHERE Field = NULL';
        return Database.getQueryLocator(query);
    }
    global void execute(Database.BatchableContext BC, List<!!ObjectType!!> records) {
        update records;
    }
    global void finish(Database.BatchableContext BC) {}
}

EmptyUpdateBatch batchy = new EmptyUpdateBatch(); 
Id batchId = Database.executeBatch(batchy);
System.debug(batchId);