Prevent arc land from failing if there is nothing to lint

Summary:
If there is not path to lint, the lint workflow will print `No paths are
lintable. and throw an ArcanistNoEffectException`.
When running arc lint, this exception is catched and returns no error,
but when run as part of arc land this logic is missing.

Test Plan:
Delete a file then run arc land --preview; it should not fail the lint
stage.

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D5263