Recent changes to Connect's InsertField will fail to inject field on key of tombstone record

This is a regression due to the changes for KAFKA-8523.

KAFKA-8523 was backported to multiple versions, and was released into 2.2.2, 2.3.1, and 2.4.0, and will soon be released in 2.5.0.

Unfortunately, that fix always makes the `InsertField` SMT skip all tombstone records, even when using the `InsertField$Key`.

Rather than:

    private boolean isTombstoneRecord(R record) {
        return record.value() == null;
    }

the correct behavior would be:

     private boolean isTombstoneRecord(R record) {
         return operatingValue(record) == null;
     }

The method no longer detects just tombstone methods, so the code should be refactored to return the record if the operatingValue for the record (which for `InsertField$Key` is the record key and for `InsertField$Value` is the record value) is null.