(stable) Fix a possible count(null) in PHUIInfoView
PHP7.2 raises a warning about count(scalar) (GREAT!) and we have one here if the caller doesn't setErrors(...).
Test Plan: Sanity-checked usage of $this->errors.
Reviewers: amckinley
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D18964