Avoid trying to create empty DrawTarget for painting range input shadow.
After bug 1918838 we can get a way smaller DPIRatio which ends up with a
zero-sized draw target. I don't think we care much about not drawing the
shadow in that case.
Differential Revision: https://phabricator.services.mozilla.com/D228074
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1928736
gecko-commit: dd1e7d287b6651b30511ff6991e87871b7101c6a
gecko-reviewers: gfx-reviewers, bradwerth