LP#1916949: combobox: asyncSupportsEmptyTermClick fix

LP#1916949: combobox: asyncSupportsEmptyTermClick fix

This is a backport of a patch that was included as part
of LP#1904244.

Fixes situation where combobox would fetch the needed entries
in async mode, but fail to open the drop-down.

This includes code originally written by Bill Erickson with
additions by Galen Charlton to insure that static data sources
would continue to work.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Ruth Frasur <rfrasur@library.in.gov>
Signed-off-by: Bill Erickson <berickxx@gmail.com>