At the moment, select field requires that you always make a selection. There is no way to make an "optional" select field. However, there are a few important reasons why this option should be available for select fields:
1) In a use case where the user is not required to make a selection from dropdown (optional select field). It is not possible at the moment.
2) It is considered a bad practice for UI forms to contain pre-selected values. At the moment, one of available options is always selected by default. Consequently, there is high probability that a user will save the form without noticing the default select field option.
3) In certain circumstances, there is legal requirement not to pre-select one of the available options in a dropdown. This cannot be met at the moment.
My suggestion is to add a new boolean property in select field, eg. "empty" with true as default, in which case the select field would default to empty value. In this case, I would expect when users don't make any selection, no value is saved in JCR (likewise when an optional text field is empty).
I'm attaching two image with the expected result.