Refactoring: simplify length comparisons

Something we often do is check whether a list or sequence has elements before we try and do something with it.

A Pythonic way of doing this is just to use the fact that Python lists and sequences evaluate to True if they have elements, and False otherwise.

Doing it this way is a convention, set out in Python's PEP8 style guide.

See for reference: https://docs.sourcery.ai/Reference/Python/Default-Rules/simplify-len-comparison/