Fix regression when pasting binary data from clipboard

Summary:
Commit b73355da3f broke pasting of binary data when the destination does
not already exist. If the StatJob fails, we need to use the URL provided
as user input, rather than an empty URL.

Test Plan: Paste something from clipboard in dolphin.

Reviewers: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13342