[SVG] SVGViewSpec.*String should return empty string when |contextElement| went away

This CL adds missing null check to SVGViewSpec::{preserveAspectRatio,viewBox}String().

BUG=345042
NOTRY=true

Review URL: https://codereview.chromium.org/176773004

git-svn-id: svn://svn.chromium.org/blink/trunk@167660 bbb929c8-8fbe-4397-9dbb-9b2b20218538