TL;DR: If there’s only one implementation of your interface, throw that interface away.
Here’s another issue that came up in a code review at work: We have an interface which has a single implementation. We expect to change that…
TL;DR: Use empty collections, not nulls.
This question came up the other day at work during a code review, so I thought this might be a good opportunity to spell out my reasons for preferring empty collections over null collection references.