Service Workers/Fetch: Fix Response constructor for empty/null

The fetch spec[1] has the body parameter of the Response constructor
marked optional, and none of the accepted types are nullable. In contrast,
the Blink implementation supported passing null but required an argument.
Make the impl match the spec.

[1] https://fetch.spec.whatwg.org

BUG=412027

Review URL: https://codereview.chromium.org/701683005

git-svn-id: svn://svn.chromium.org/blink/trunk@185051 bbb929c8-8fbe-4397-9dbb-9b2b20218538