Fix a possible count(null) in PHUIInfoView

Summary:
See https://discourse.phabricator-community.org/t/php7-2-exception-count-parameter-must-be-an-array-or-an-object-that-implements-countable/1031.

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