DevTools: hide autocomplete when the autocompletion prefix is empty

This patch hides autocomplete suggestion box in editor whenever
autocopmletion prefix length is zero.

BUG=358255

Review URL: https://codereview.chromium.org/219773004

git-svn-id: svn://svn.chromium.org/blink/trunk@170573 bbb929c8-8fbe-4397-9dbb-9b2b20218538