ui/PRESUBMIT.py contained a check for constructing an empty unique_ptr<>
as an rvalue. It's almost always better to use nullptr instead. However, this
check was disabled because it was overly broad, matching things like

  std::vector<std::unique_ptr<T>>()

which cannot be replaced by nullptr.

Make the check stricter so that it will not overmatch, and re-enable it.

Also fix some violations of the rule that were found during testing.

BUG=

Review-Url: https://codereview.chromium.org/2311783002
Cr-Commit-Position: refs/heads/master@{#416865}