[13.x] Add assertEmpty to the Storage facade

[jackbayliss]

Currently, asserting the whole disk is empty requires passing an empty string to assertDirectoryEmpty which reads oddly / feels a bit like a hack.

This PR adds an assertEmpty method, which is fully recursive, so it fails if a file exists anywhere on the disk.

Storage::fake('some-specific');

// Tests....


// Before
Storage::assertDirectoryEmpty(''); // wut wut?!

// After
Storage::assertEmpty();

We have a lot of tests that set specific disks up, and we need to make sure they're empty ie certain files are moved / deleted etc via jobs, commands, etc...

I can macro it worst case, but feels like something everyone may benefit from.

Perhaps you prefer assertDiskEmpty - so open to changing!

[github-actions]

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