Add ability for empty dropdown option

[markcrafted]

Ability to have an empty dropdown value prompting the user to select an option.

For example "Please Select" as an option with a null value.

You can't create a dropdown with a caption with an empty value as it is saved with a value of 'undefined'.

Another option would be to have validation on the dropdown value. Then a user can create an option "Please Select" with a value but cause the control to validate against it.

[AndyButland]

This will be included in the next minor release for Forms.