Database / Form Select general question

So this question is really just a very generalised one, just for my own brain to make sure I am doing things correctly.

Here is the scenario
I have a form on my page that has a text field input and a select element and a submit button
text field is not required
select is not required options (Please Select - value="")(Yes - value=“yes”)(No - value=“no”)

I have a database that only takes these two entries, the first field in the database is set to varchar(100) and the second is set to enum(“yes”,“no”) and both allow NULL.

Now if I inserted the form to the database with no fields entered at all, just for testing purposes, why does the varchar have NULL inserted while the enum from the select is just blank, why is the value not equal to NULL rather, do I have to set the value of Please Select to NULL manually?

Just wondering how others handle this type of scenario.