Assuming you're working from the Data tab of an Item page in AGOL, the SQL coalesce function might do what you're looking for. It accepts multiple arguments and returns the first non-null argument. For example, if you have a field named Label where some records have labels (and you want to keep those) but you want to use 'Feature (objectid)' for those with null values, you could do something like this:
coalesce(LABEL, CONCAT('Feature ', OBJECTID))
Anything with a label will get the first value (the existing value) and anything that's null will get "Feature #".
I would recommend testing this on some data that you don't mind messing up first however, like a temporary field added for this purpose.