Set blank vale as default for a select box using Drupal form API?
In Drupal 7, you could use the #empty_option property, for which the default value is "- Select -" for a required field, and "- None -" for an optional field. The property adds the additional label as per the following cases:


If #required is TRUE and there is no #default_value, an empty option is added to the select control to force the user to make an active choice.
If #empty_value or #empty_option is set and #required is FALSE (default), an empty option is added to the select control, allowing the user to choose nothing.
If none of #required, #empty_value, #empty_option, and #default_value are set, then no empty option is added to the select control. This leaves the control in a slightly illogical state, since all user agents automatically preselect the first available option. There's no way for the user to select nothing, and the user is also not forced to make an active decision.


The property is used only for "select" form fields.