Minor refactoring in FormatBlockCommand::formatRange.
Do an early return if root and refNode elements are null. This shall help avoid making some unnecessary calls such as splitTreeToNode and creating range.
No new tests are required for this change.

Review URL: https://codereview.chromium.org/96653002

git-svn-id: svn://svn.chromium.org/blink/trunk@162891 bbb929c8-8fbe-4397-9dbb-9b2b20218538