[BUGFIX] Context menu for offline storages raises exception

If you try to call the context menu for an offline storage an exception is
raised. Therefore an offline storage shouldn't be linked with a context
menu.

Change-Id: I2845d173bd2a22c96e23bb58bf1a57239109829a
Fixes: #46551
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/20195
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack