[Controls] Fix deselecting (blank) option from options list (elastic#242036)

## Summary

Closes elastic#241750

Fixes an error with deselecting a `(blank)` option from an options list.

The same error when *selecting* the option was fixed in
elastic#239791, but we missed fixing it
for deselection.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

(cherry picked from commit 783f287)

Author: Zacqary

src/platform/plugins/shared/controls/public/controls/data_controls/options_list_control/get_options_list_control_factory.test.tsx

@@ -269,6 +269,11 @@ describe('Options List Control Api', () => {
       await waitFor(async () => {
         expect(option).toBeChecked();
       });
+
+      await userEvent.click(option);
+      await waitFor(async () => {
+        expect(option).not.toBeChecked();
+      });
     });
 
     test('clicking another option unselects "Exists"', async () => {

src/platform/plugins/shared/controls/public/controls/data_controls/options_list_control/get_options_list_control_factory.tsx

@@ -345,7 +345,7 @@ export const getOptionsListControlFactory = (): DataControlFactory<
         loadMoreSubject,
         deselectOption: (key: string | undefined) => {
           const field = api.field$.getValue();
-          if (!key || !field) {
+          if (key == null || !field) {
             api.setBlockingError(
               new Error(OptionsListStrings.control.getInvalidSelectionMessage())
             );