D3D: Return an empty string when the translated shader source is empty. TEST=conformance/extensions/webgl-debug-shaders BUG=angleproject:1991 Change-Id: Id210ec43501c4e4f3db2c8a0aec25e1d20b18d33 Reviewed-on: https://chromium-review.googlesource.com/475992 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>