[iOS 14] Handle nil assetCollection and prevent crash

[frosty]

This PR is an alternative to #360, to fix wordpress-mobile/WordPress-iOS#14958. This fixes the issue by checking for a nil asset collection.

To test

• Build and and run the associated WPiOS branch on iOS 14
• Visit the Media Library, choose +, tap Choose From My Device, then Select More Photos (or Select Photos if this is the first time you've run the app). Ensure that no photos are selected and tap Done.
• The media picker should be displayed with no content, and the app should not crash.

[diegoreymendez]

Hey @frosty I just realized the tests are broken apparently.

[frosty]

I just ran the tests locally and they seem to be fine. I think the issue is just the podspec hash, so it should be fine when I get everything merged and the pod released.

[diegoreymendez]

👍