[Response Ops][Cases] Spinner on assigning user to case from case page (

#191490)

## Summary

Fixes #190303

In 8.15 we disabled a request that showed a spinner while React Query's
`isLoading` was true. `isLoading` remains true as long as no data has
been received. Because the request was disabled, `isLoading` stayed true
the entire time, causing the spinner to be displayed instead of the form
that would have activated the request. The fix uses another React Query
state called `isFetching`, which is only true while the request is
actively being made, so initially, no spinner is shown.

## Release Note
Fix endless loading spinner when wanting to add a assignee to a case
from the cases list

x-pack/plugins/cases/public/components/actions/assignees/edit_assignees_selectable.test.tsx

@@ -460,4 +460,18 @@ describe('EditAssigneesSelectable', () => {
       ],
     });
   });
+
+  it('renders even with no assignee set yet', async () => {
+    const selectedCases = [{ ...basicCase, assignees: [] }];
+    const result = appMock.render(
+      <EditAssigneesSelectable {...props} selectedCases={selectedCases} />
+    );
+
+    await waitFor(() => {
+      expect(result.getByTestId('cases-actions-assignees-edit-selectable')).toBeInTheDocument();
+    });
+
+    expect(result.getByPlaceholderText('Find a user')).toBeInTheDocument();
+    expect(result.getByText('Selected: 0')).toBeInTheDocument();
+  });
 });

x-pack/plugins/cases/public/components/actions/assignees/edit_assignees_selectable.tsx

@@ -79,7 +79,7 @@ const EditAssigneesSelectableComponent: React.FC<Props> = ({
     [selectedCases]
   );
 
-  const { data, isLoading: isLoadingUserProfiles } = useBulkGetUserProfiles({
+  const { data, isFetching: isLoadingUserProfiles } = useBulkGetUserProfiles({
     uids: Array.from(assignees.values()),
   });