How to run a search with undefined token

Hello, I have a panel on a dashboard with a search using a token to exclude events with some field values. Search is something like `index=myindex sourcetype=mysourcetype $exclude$` The "exclude" token is generated from a checkbox input which generate a string like "myfield!=value1 myfield!=value2 ..." This is working fine if at less one entry on checkbox is selected but a have the message `Search is waiting for input... `if no entries are selected. I open the panel in search and see `undefined` on search bar. Is it possible to run the search even if the token is not defined or to have an empty defined token if all entries on checkbox are unchecked? Thanks