Fixes search in macOS example, close #371

Summary:
Changed the criteria for starting search in the OS X Example. Closes https://github.com/Instagram/IGListKit/issues/371

Doesn't look like there are tests for the example app. Please ping me if you'd like me to add some. Also, please ping me if you'd like me to add this change to the changelog -- I'm assuming bug fixes to sample apps don't get included there.

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/377

Differential Revision: D4374006

Pulled By: jessesquires

fbshipit-source-id: 6e20f61aece769240aa63c8949d3c06a0ecd26c8

Examples/Examples-macOS/IGListKitExamples/ViewControllers/UsersViewController.swift

@@ -34,7 +34,7 @@ final class UsersViewController: NSViewController {
     }
     
     private func computeFilteredUsers() {
-        guard searchTerm.characters.count > 1 else {
+        guard !searchTerm.characters.isEmpty else {
             filteredUsers = users
             return
         }