Treat empty string as title-set in ToolbarWidgetWrapper

If a developer explicitly sets the empty string as the title of a
Toolbar that is used as an action bar via Activity#setActionBar, don't
overwrite it with the window title. This clears up a nonintuitive
behavior when explicitly clearing the title of a toolbar.

Bug 17510951

Change-Id: I183c2ca57d3bd16ae1f89bb7daa2cdf086189138