fix: prevent crash when mobile workspace list is empty

[hitesh358-str]

Summary

This PR prevents a crash in MobileWorkspaceStartScreen when the workspace list is empty.

Root Cause

The Get Started button accessed:

widget.workspaceState.workspaces.first

when selectedWorkspace == null.

If the workspace list was empty, this caused:

Bad state: No element

Changes

• Added a guard to check whether workspaces exist
• Disabled the Get Started button when workspace list is empty
• Preserved existing behavior when workspaces are available
• Added widget test coverage for the empty workspace state

Testing

• Verified normal flow when workspace exists
• Verified button becomes disabled when workspace list is empty
• Verified no .first access occurs on empty lists

PR Checklist

• My code adheres to AppFlowy's Conventions
• I've listed at least one issue that this PR fixes in the description above.
• I've added a test(s) to validate changes in this PR, or this PR only contains semantic changes.
• All existing tests are passing.

Summary by Sourcery

Prevent MobileWorkspaceStartScreen from crashing when the workspace list is empty and add coverage for the empty state.

Bug Fixes:

• Disable the Get Started action when there are no workspaces to avoid accessing the first element of an empty list.

Tests:

• Add a widget test ensuring the Get Started button is disabled when the workspace list is empty.

[CLAassistant]

All committers have signed the CLA.

[sourcery-ai]

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[sourcery-ai]

Hey - I've left some high level feedback:

• In the widget test, consider using a more specific finder (e.g., by text or a key) instead of find.byType(ElevatedButton) to avoid brittleness if additional buttons are added to the screen in the future.
• The onPressed logic in MobileWorkspaceStartScreen’s ElevatedButton is starting to get a bit dense; consider extracting the workspace-selection and _popToWorkspace decision into a small helper method to keep the build method easier to scan.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In the widget test, consider using a more specific finder (e.g., by text or a key) instead of `find.byType(ElevatedButton)` to avoid brittleness if additional buttons are added to the screen in the future.
- The `onPressed` logic in `MobileWorkspaceStartScreen`’s `ElevatedButton` is starting to get a bit dense; consider extracting the workspace-selection and `_popToWorkspace` decision into a small helper method to keep the `build` method easier to scan.

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