IsEmpty( YourTable::yourfield )
only checks the contents of yourfield for the current record in YourTable--assuming the layout is based on YourTable.
In either a scripted or manual find, you can find all records where a given field is empty by putting the = operator in it with no other criteria specified in that field.
You can also use Replace Field contents to give all such records the same value. Since replace field contents modfies all records in your found set and the changes cannot be undone, it's a good idea to read up on this tool in FileMaker help first, then make a back up copy of your file before trying it out.