Show an empty state on the All Companies tab

**Problem**

The All Companies tab handles a null list but has no empty-list branch, so it renders a blank white screen. Its sibling tab handles this correctly.

This is visible right now: the backend returns an empty queryset for any candidate whose season is not active, which with the season over is currently every student.

**Where**

- `lib/views/ProfilesForAllView.dart:35-38` (null branch only, no `isEmpty`)
- `lib/views/ProfilesForMeView.dart:35-40` (correct: has the `isEmpty` branch)
- Backend: `omniport-app-placement-and-internship/utils/filters.py:14-22`

**Expected**

"No companies open right now" rather than a blank page.

**Fix sketch**

- Add the `isEmpty` branch, matching `ProfilesForMeView`.

Related to #28, which is about the wording of the sibling tab's empty message.

Compare lib/views/ProfilesForAllView.dart:35-38 with the isEmpty branch in lib/views/ProfilesForMeView.dart:35-40. Add the matching empty-list state to the All Companies tab, using the expected "No companies open right now" message. Verify that an empty backend result no longer renders a blank page.

Written by the indexing model from the issue text.