Is there a way to make parameters on an expression rule optional?...

Parameters on an expression rule are technically always optional - if you use the [parameterName]: to define your parameters, the process/rule will always pass in null for any parameters skipped. As Miguel has stated, there is a requirement to pass in at least one parameter if parameters exist in the expression rule, but if that's a problem for whatever reason, you can create a dummy variable that does nothing within your rule.

Parameters on an expression rule are technically always optional - if you use the [parameterName]: to define your parameters, the process/rule will always pass in null for any parameters skipped. As Miguel has stated, there is a requirement to pass in at least one parameter if parameters exist in the expression rule, but if that's a problem for whatever reason, you can create a dummy variable that does nothing within your rule.