Revert "remove unnecessary code from Connection::insert"

This reverts commit 5b86fad8740135ed2cfee9b6da0b7ddc50f8a066 which was breaking inserts
without explicitly set values. This can happen when using an auto increment primary key
in conjunction with all other columns having a default. The insert should still be possible
in this case, even if the row just contains defaults or null values afterwards.