Allow end user to define a label for the empty option in a select

[anubisthejackle]

Exactly as it says. This PR adds a parameter "first_empty_label" that allows changing of the displayed name for empty options. The value of this option remains empty.

[renatonascalves]

🗡️

[dlh01]

This seems useful to me. A unit test for this behavior would be helpful alongside the other first_empty-related tests: https://github.com/alleyinteractive/wordpress-fieldmanager/blob/1994637fbe962b94c943d0dab591ab75609f605e/tests/php/test-fieldmanager-select-field.php. I would also offer that it might be beneficial for the property to be [something]_label, like first_empty_label, for greater consistency with other field properties like add_more_label and button_label.

[anubisthejackle]

@dlh01 You're right. I've updated the label name and added a test for this.