Requiring unique values on non-mandatory fields should allow empty values too

If I attempt to set a textfield to true for "unique" but also set it to false for "mandatory" I would expect it to only validate values other than empty strings. However, it validates everything including an empty string.

End result: only 1 submission of this form can be performed with this "non-mandatory" field, all other submissions will state that

the value "" has already been submitted once for the [non-mandatory] field.