Use `isEmpty` for Iterables and MapsDRT-W1147

DON'T use length to see if a collection is empty.

The Iterable contract does not require that a collection know its length or be able to provide it in constant time. Calling length just to see if the collection contains anything can be painfully slow.

Instead, there are faster and more readable getters: isEmpty and isNotEmpty. Use the one that doesn't require you to negate the result.

BAD:

if (lunchBox.length == 0) return 'so hungry...';
if (words.length != 0) return words.join(' ');

GOOD:

if (lunchBox.isEmpty) return 'so hungry...';
if (words.isNotEmpty) return words.join(' ');