QUrl: fromLocalFile(QString()) should lead to an empty URL.

This is much more useful than the URL "file:", it allows to use
"empty path" and "empty URL" for the same meaning (e.g. not set).

QFileDialog actually uses "file:" though, as the URL for the
"My Computer" item in the sidebar. This patch preserves that.

[ChangeLog][QtCore][QUrl] QUrl::fromLocalFile now returns an empty URL
if the input string is empty.

Change-Id: Ib5ce1a3cdf5f229368e5bcd83c62c1d1ac9f8a17
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>