Fix #20724 - Disable "Hide/show naturals" in keysig context menu

Fix #20724 - Disable "Hide/show naturals" in keysig context menu when unnecessary.

When the score is set up for hiding naturals in key sig changes, the "Hide / show naturals" in key sig contextual menu seems useless.

Fixed by disabling the contextual menu item when that score setting is set or when the key sig change has no naturals.