Fix incorrect extension for tempfile

Fix incorrect extension for tempfile

I set ext to None in commit f2a70843905839a1b11795310d2aa3b85a474e77
without realizing _generate_tempfile does a simple string formatting.
A simple test proves this does not generate an empty string:

$ python -c "f = '%s' % None; print f, type(f)"
None <type 'str'>

So pass an empty string instead of None for the suffix parameter.

https://bugzilla.gnome.org/show_bug.cgi?id=659502