One more thing this requirement is there because we have more than 100 standard public reports and there are many users who have only read access.
In many report we have used universe level conditions( for which we dont have facility to set it as a optional ) , when user want to run a specific report for all countries in that case they have to manually select all countries, hence what we are trying to do if he wants data for all counties then should simply not select any value
Universe Filter Condition Name : Select Country
@Select(Asset\Country) IN @Prompt('Enter Asset Country(s)','A','Asset\Dataset ID',MULTI,FREE)