[5.5] Added isNotEmpty to complement isEmpty

[timacdonald]

I've come to love the isEmpty and isNotEmpty methods on the collection class.

It's always great when you can guess what methods are available in Laravel - and they are there.

Now that I've grown accustomed to the isEmpty method - I assume everywhere I can use that, I can use the complement isNotEmpty method. Unfortunately this is not standard across the code base. For example on the MessageBag class there is an isEmpty and an any method. This PR introduces an isNotEmpty method where ever the isEmpty method is present.

I'm not 100% sure if this should change the contracts, but as 5.5 is nearly here, I thought I'd add it to the contracts and the classes anyway and pull it into 5.5.

I haven't added any tests, would rather know if this is wanted / appropriate first.