If Sketchbook is empty, no Sketchbook menu item is present under File

[ubidefeo]

Describe the problem

This happens in two occurrences:

1. If you have a brand new installation there is no Sketchbook item under File
2. If you point to an empty folder as your Sketchbook location and restart the IDE without saving a single Sketch, when you relaunch the IDE you can observe the same behaviour

To reproduce

1. Select File > Preferences from the Arduino IDE menus.
2. Change the "Sketchbook location preference to some folder that does not contain sketches.
3. Click the OK button.
4. Select File > New from the Arduino IDE menus.
   ⓘ This is necessary to workaround a bug where the IDE doesn't update the menus after the preference change: IDE UI does not reflect changed "Sketchbook location" preference #796
5. Select File from the Arduino IDE menus.

🐛 There is no File > Sketchbook menu:

Expected behavior

The "Sketchbook" item should always be there even if empty, as it works in Arduino IDE 1.8.x:

Arduino IDE version

2.0.0 Beta 12

Operating system

macOS (probably any OS)

Operating system version

Big Sur

Additional context

Related: #655

Issue checklist

• I searched for previous reports in the issue tracker
• I verified the problem still occurs when using the latest nightly build
• My report contains all necessary details