Comments search should only filter the main query or ignored for the To Do widget

[ocean90]

o2_Search uses query filters which affects all queries, even those by the To Do widget. Because of this the widget query may not return any results.

Example: https://make.wordpress.org/polyglots/?s=notfound
The widgets "Editor Requests", "Locale Requests" and "General Requests" are empty.

A fix would be to filter only the main query or to set $args['s'] = '' for the widget query.