IResource#getLocation() returns null for virtual resources that do not
exist in the local file system.

Fall back to copying input to output stream like before
9c6c5519dfb245b2b1c9bf5b3686cf9c324bd67e [1] in this case.

Fixes #615.

[1] https://bugs.eclipse.org/bugs/show_bug.cgi?id=549486