Here are the steps to reproduce the following bug:

1. Download a Russian language pack from this site
2. Install it and switch the language to Russian
3. Create a new user using Russian character with the username of (привет) which means 'hello'
4. In Confluence search box, search for any strings/characters
5. You will see no errors
6. Then try to insert a user which does not exist in Confluence (тест) which means 'test' in the Author/Who section
7. Click on Filter and you will get the following error message:

   ...
   java.lang.IllegalArgumentException: query String must be specified
   	at com.atlassian.confluence.search.service.DefaultPredefinedSearchBuilder.buildUsersSearch(DefaultPredefinedSearchBuilder.java:41)
   	at com.atlassian.confluence.search.actions.SearchSiteAction.getContributors(SearchSiteAction.java:283)
   	at com.atlassian.confluence.search.actions.SearchSiteAction.validate(SearchSiteAction.java:170)
   ...
   

If the user exist in Confluence, this error message will not be thrown.