Support displaying footer on empty options for <a-select>

[FerricIon]

• [x] I'm sure this does not appear in the issue list of the repository

Basic Info

What are the similar cases of this feature

None.

What problem does this feature solve?

Currently footer slot will not be shown on empty options. If a filter is put in the footer and a user updates the filter resulting in empty options, the filter will disappear. In that case no more actions could be made, and the options are gone forever. A boolean prop can be added to to support displaying footer on empty options.