[webcrypto] Interpret empty BigInteger as 0 according to the spec

BUG=373552
R=eroman
Tests= crypto/ LayoutTests

Review URL: https://codereview.chromium.org/595873003

git-svn-id: svn://svn.chromium.org/blink/trunk@183826 bbb929c8-8fbe-4397-9dbb-9b2b20218538